Beyond what we can see with our eyes, that is, the visual spectrum (and I suppose, for a few, ghosts in the spirit world), humans have been quite adept at creating 'artificial' eyes to help us see further. Depending on the tool used, we can now see infrared waves, ultraviolet waves, radio waves, x-rays, etc. The common denominator here is that all of these waves exist in the electromagnetic spectrum.
Last year, we created a tool which allows us to "see" into the gravitational wave spectrum.
Now, we can "see" black holes or other such phenomena - that is, if they are sufficiently powerful enough to produce gravitational waves potent enough for us to detect them. Step by step, we are slowly able to understand the universe.
Want to learn more? Click here to read my article on this topic, complete with historical context and graphics