Attempts to find out what's beyond our galaxy
So, now we know that there are millions of other stars in various galaxies around us. What we don't know is whether the universe is limited or not. What lies beyond the marvelous nebula clouds, shiny stars, unexplored planets? NOTHING! Nothing is what it would seem to be at a glance. No galaxies, no stars, just blackness. No wonder some people are afraid of the dark -- you could say it's the ultimate end! But what I just said is only what's in the realm of our understanding. We found out that our Sun is not the only star in the galaxy, we found out that our galaxy is not the only one. Maybe now we'll found out that beyond the darkness, an another cluster of galaxies lies. Maybe an another big bang took place far far away. Come to think of it, maybe our big bang was a small part of a much bigger explosion, and planets the size of our solar system loom somewhere, behind the darkness. But now - onto the dark matter.
The image above shows dark matter(blue) that split from the luminous matter (red). This is the only way that dark matter can be seen, as it is not visible by any telescopes. Why is dark matter invisible? Because it's different from any other atomic matter, that we're used to seeing around us. Dark matter does not reflect light or emit heat. So how did it receive a color? Well, the observed galaxies were passing through one another at a speed of 10 million miles per hour. The luminous matter in both galaxies interacted with each other, and slowed down. The dark matter, on the other hand, did not react whatsoever, thus remaining at it's full speed. That's how the matters separated. Quite astonishing, as the chances of seeing this happen at one point are probably low.
Right then. One thing's clear: dark matter exists. But what is it exactly? It can't be nothing, can it? Scientists believe that the dark matter is a gas of extremely low density. Recently, an x-ray image of several galaxies was taken, in an attempt to find hot spots of dark matter. Only 5% of the known universe is made from matter we know (atoms with electrons and protons). The rest is taken by dark matter and dark energy. Despite the search field being potentially limitless, scientists managed to find a hot-spot of the dark matter. This was a groundbreaking discovery, as now we know what fills the huge empty voids between galaxies.
That's one more unknown thing crossed out of the list. As ancient people thought that lightning is the wrath of God, so we figured that the dark matter is nothing. Thanks to constant study and observation, now we're one step further in researching the universe. Who knows, maybe we'll find out that it has an end.