Dark matter is... dark, as in something you can't see, as in something we are clueless about.
The idea of dark matter is not anything new. Isaac Newton knew about it. He knew that there is more to the universe than meets the eye. Apparently, all the billions of galaxies we see through telescopes is only a small portion of the stuff in the universe. The rest of the stuff is something we can't see, hear, or detect in other ways.
We call it dark matter. But it's not even matter as we know it. It's made up of something else. We know it's there because when astronomers added up the mass of all the visible objects in the universe, they found out there's a lot of missing mass we can't account for. This missing mass could be what prevents our universe from collapsing onto itself.
Isaac Newton called it God. Scientists since Isaac Newton tried to explain it away with some sort of a constant. But nobody knows for sure. A Bible verse from Collosians 1:17 comes to mind now that I think about it:
"He existed before anything else,
and he holds all creation together." - Colossian 1:17
Could that be referring to dark matter? Was Isaac Newton right? I don't know. But since nobody else came up with an answer yet, I have no problem sticking with that explanation for now.
P.S. Dark matter is a fudge factor that has been invoked whenever scientists couldn't explain something. At the current state of physics, dark matter is invoked to explain the missing mass in a universe ruled by the force of gravity. But some scientists suggest the electro-magnetic force, and not gravity, is the dominant force in the universe. In the electric universe, there is no missing mass, and the existing phenomena in the universe can be explained as a result of electro-magnetic forces.