Q: What does "hang out" mean in this specific sentence In a book that I'm reading there is a sentence like this:

there is a great mystery hanging out in the background.

Here is the context:

What are the smallest bits and how do they work together to make the universe? But even as we grasp for answers to questions about the tangible things surrounding us, there is a great mystery hanging out in the background. That mystery is the background itself: space.

My question is what does "hanging out (in the background)" mean in this sentence? 
Does it bear the original meaning of hang, i.e to put or fix, or to be put or fixed, above the ground?

A: I am thinking it just means "there is a great mystery in the background, which we are not focusing much on". So hanging out would just mean existing in the general sense.
