<blockquote>
  <p>You cannot have dark without light.</p>
</blockquote>

<p>Not true in the case of dark matter (for the general case, see below). Dark matter is called "dark" because it appears that it doesn't absorb or emit electromagnetic radiation - light! It can interact with light via gravitational lensing, but dark matter particles have no electromagnetic charge (we think) and so are considered "dark".</p>

<blockquote>
  <p>The only thing faster than light is dark (i.e. shadows)</p>
</blockquote>

<p>This is true, as the video explains.</p>

<blockquote>
  <p>Does this make "dark matter" mere shadows of extremely bright objects?</p>
</blockquote>

<p>This is the meat of the question, and the answer is a clear "no".  First, any "bright objects", while capable of <a href="https://physics.stackexchange.com/questions/117101/can-the-sun-cast-a-shadow">casting a shadow of something <em>else</em></a>, will emit electromagnetic radiation.</p>

<p>Okay, so why can't there just be non-extremely luminous objects casting a shadow? One reason is that the dark matter simply isn't where these shadows should be. Many galaxies have a <a href="http://en.wikipedia.org/wiki/Dark_matter_halo" rel="nofollow noreferrer">dark matter halo</a> that extends far beyond the galactic plane. Any light sources emitting light (that is then shadowed) would have to be on the opposite side of the galactic disk. This is simply not the case.</p>

<p>The second, more convincing, reason is that dark matter has mass. In fact, that's the reason the idea was first conceived (to account for anomalies in <a href="http://en.wikipedia.org/wiki/Galaxy_rotation_curve" rel="nofollow noreferrer">galactic rotation curves</a>)! Shadows don't have mass; therefore, dark matter cannot be explained by shadows.</p>

<blockquote>
  <p>Is that also why it has to get dark before we can really see the majority of stars (subjective experience)?</p>
</blockquote>

<p>No. I don't know for sure, but I believe that that's because the Sun's light is still blocking out the light of distant stars.<sup>[citation needed]</sup></p>

<hr>

<p><strong>Addendum after edit to question</strong></p>

<blockquote>
  <p>Can you provide sources for your claims?</p>
</blockquote>

<p>I think that they're pretty well sourced! The first section can easily be checked via <a href="https://en.wikipedia.org/wiki/Dark_matter" rel="nofollow noreferrer">Wikipedia</a> (which I assumed you had seen). The rest seems to be well-sourced, too (aside from the last bit, which is, admittedly, unsourced but only tangential).</p>

<blockquote>
  <p>Also I didn't say you can't have DARK MATTER without Light , I said you can't have Dark, as in Darkness. I think you're getting hung up on Dark Matter which is a big part of my question, but not the entire question.</p>
</blockquote>

<p>Okay, I may have misinterpreted this.</p>

<p>Darkness is, by definition, the absence of light (electromagnetic radiation). So no, you don't have to have light for there to be darkness.</p>

<blockquote>
  <p>Also, are you claiming that all darkness in space is dark matter? I don't like to assume but for scientific purposes (not an accredited scientist) would we not agree that the measures made on dark matter were on a sample, and not the entire universe?</p>
</blockquote>

<p>I never said that, but you did. Your original question asked if dark matter could be shadows. The answer is no.</p>

<blockquote>
  <p>So to further question this, do shadows not exist in space?</p>
</blockquote>

<p>They exist. Any object blocking a light source casts a shadow:</p>

<p><a href="https://upload.wikimedia.org/wikipedia/commons/2/2e/Diagram_of_umbra%2C_penumbra_%26_antumbra.png" rel="nofollow noreferrer"><img src="https://upload.wikimedia.org/wikipedia/commons/2/2e/Diagram_of_umbra%2C_penumbra_%26_antumbra.png" alt=""></a></p>

<p>This happens to have been used in the video, so it shouldn't be new to you.</p>

CC BY-SA 3.0
2015-03-17T19:15:15.643Z
Interesting response. I watched this video, what do you think of this?https://youtu.be/JTvcpdfGUtQ
Hituptony
CC BY-SA 3.0
2015-03-19T00:08:09.963Z
Shadows (since they're not material objects) certainly can travel faster than light. Consider a light source at position 0, an object 1 foot away (1 light-nanosecond) casting a shadow, and a wall 100 feet away. Move the object laterally at 0.1c. It will take 100 nanoseconds for the shadow to start moving, but it will then move at 10c. Set up the angles right, and it can move infinitely fast. A shadow is not a rod or a pair of scissors.
Keith Thompson
CC BY-SA 3.0
2015-03-19T00:14:01.333Z
Wouldn't that imply . . . . never mind, I think I understand now.
HDE 226868
CC BY-SA 3.0
2015-03-17T15:45:03.447Z
2015-03-19T02:36:45.573Z
2017-04-13T12:40:15.377Z
HDE 226868
<p>Perhaps my title is misleading. but here is my understanding and I'm looking for confirmation.</p>

<p>You cannot have dark without light.</p>

<p>The only thing faster than light is darkness (i.e. shadows)</p>

<p>Stars are in the galaxy and are inherently bright.</p>

<p>Does this make "dark matter" mere shadows of extremely bright objects?</p>

<p>Is that also why it has to get dark before we can really see the majority of stars (subjective experience)?</p>

<p><strong>Update:</strong></p>

<p>With all due respect to user with answer below and thank you for taking the time to answer... I think this can be expanded on.</p>

<p>I have provided a source for my claims: youtu.be/JTvcpdfGUtQ </p>

<p>Check out the video, and it more explains my understanding of things...</p>

<p>Why I don't accept answer below:</p>

<p>Can you provide sources for your claims?</p>

<p>Also I didn't say you can't have DARK MATTER without Light , I said you can't have Dark, as in Darkness. I think you're getting hung up on Dark Matter which is a big part of my question, but not the entire question.</p>

<p>Also, are you claiming that all darkness in space is dark matter? I don't like to assume but for scientific purposes (not an accredited scientist) would we not agree that the measures made on dark matter were on a sample, and not the entire universe?</p>

<p>So to further question this, do shadows not exist in space?</p>

<light><dark-matter>
Shadows of Light = Space or Dark Matter?
Hituptony
# Shadows of Light = Space or Dark Matter?

(Asked by: username_0 on 2015-03-17)

Perhaps my title is misleading. but here is my understanding and I'm looking for confirmation.

You cannot have dark without light.

The only thing faster than light is darkness (i.e. shadows)

Stars are in the galaxy and are inherently bright.

Does this make "dark matter" mere shadows of extremely bright objects?

Is that also why it has to get dark before we can really see the majority of stars (subjective experience)?

**Update:**

With all due respect to user with answer below and thank you for taking the time to answer... I think this can be expanded on.

I have provided a source for my claims: youtu.be/JTvcpdfGUtQ

Check out the video, and it more explains my understanding of things...

Why I don't accept answer below:

Can you provide sources for your claims?

Also I didn't say you can't have DARK MATTER without Light , I said you can't have Dark, as in Darkness. I think you're getting hung up on Dark Matter which is a big part of my question, but not the entire question.

Also, are you claiming that all darkness in space is dark matter? I don't like to assume but for scientific purposes (not an accredited scientist) would we not agree that the measures made on dark matter were on a sample, and not the entire universe?

So to further question this, do shadows not exist in space?

**Tags:** light, dark-matter


---

## 1 answers

### Answer by username_1 on 2015-03-17. Score: 5 | Accepted answer

> You cannot have dark without light.

Not true in the case of dark matter (for the general case, see below). Dark matter is called "dark" because it appears that it doesn't absorb or emit electromagnetic radiation - light! It can interact with light via gravitational lensing, but dark matter particles have no electromagnetic charge (we think) and so are considered "dark".

> The only thing faster than light is dark (i.e. shadows)

This is true, as the video explains.

> Does this make "dark matter" mere shadows of extremely bright objects?

This is the meat of the question, and the answer is a clear "no". First, any "bright objects", while capable of [casting a shadow of something *else*](https://physics.stackexchange.com/questions/117101/can-the-sun-cast-a-shadow), will emit electromagnetic radiation.

Okay, so why can't there just be non-extremely luminous objects casting a shadow? One reason is that the dark matter simply isn't where these shadows should be. Many galaxies have a [dark matter halo](http://en.wikipedia.org/wiki/Dark_matter_halo) that extends far beyond the galactic plane. Any light sources emitting light (that is then shadowed) would have to be on the opposite side of the galactic disk. This is simply not the case.

The second, more convincing, reason is that dark matter has mass. In fact, that's the reason the idea was first conceived (to account for anomalies in [galactic rotation curves](http://en.wikipedia.org/wiki/Galaxy_rotation_curve))! Shadows don't have mass; therefore, dark matter cannot be explained by shadows.

> Is that also why it has to get dark before we can really see the majority of stars (subjective experience)?

No. I don't know for sure, but I believe that that's because the Sun's light is still blocking out the light of distant stars.[citation needed]

---

**Addendum after edit to question**

> Can you provide sources for your claims?

I think that they're pretty well sourced! The first section can easily be checked via [Wikipedia](https://en.wikipedia.org/wiki/Dark_matter) (which I assumed you had seen). The rest seems to be well-sourced, too (aside from the last bit, which is, admittedly, unsourced but only tangential).

> Also I didn't say you can't have DARK MATTER without Light , I said you can't have Dark, as in Darkness. I think you're getting hung up on Dark Matter which is a big part of my question, but not the entire question.

Okay, I may have misinterpreted this.

Darkness is, by definition, the absence of light (electromagnetic radiation). So no, you don't have to have light for there to be darkness.

> Also, are you claiming that all darkness in space is dark matter? I don't like to assume but for scientific purposes (not an accredited scientist) would we not agree that the measures made on dark matter were on a sample, and not the entire universe?

I never said that, but you did. Your original question asked if dark matter could be shadows. The answer is no.

> So to further question this, do shadows not exist in space?

They exist. Any object blocking a light source casts a shadow:

[![](https://upload.wikimedia.org/wikipedia/commons/2/2e/Diagram_of_umbra%2C_penumbra_%26_antumbra.png)](https://upload.wikimedia.org/wikipedia/commons/2/2e/Diagram_of_umbra%2C_penumbra_%26_antumbra.png)

This happens to have been used in the video, so it shouldn't be new to you.

> **Comments:**
> **username_0**: Interesting response. I watched this video, what do you think of this?https://youtu.be/JTvcpdfGUtQ  
> **username_2**: Shadows (since they're not material objects) certainly can travel faster than light. Consider a light source at position 0, an object 1 foot away (1 light-nanosecond) casting a shadow, and a wall 100 feet away. Move the object laterally at 0.1c. It will take 100 nanoseconds for the shadow to start moving, but it will then move at 10c. Set up the angles right, and it can move infinitely fast. A shadow is not a rod or a pair of scissors.  
> **username_1**: Wouldn't that imply . . . . never mind, I think I understand now.  

---

