I have begun reading a book called The Cosmic Cocktail by Katherine Freeze and was immediately interested into one of the first topics she discusses that helps prove the existence of dark matter.
A long time ago, or at least before I was born, Albert Einstein made his Theory or Relativity which stated many things including the fact that light will always travel at the same speed, about 3 x 10^8 meters per second, but he also stated that the gravitational pull of a planet can affect light, or bend it! This phenomena became known as Gravitational Lensing since the planets acted almost like a lense by bending the light. The major difference though was that the light is bent most closest to the body pulling it, and least farther away.
One might ask, why do I care about some bent light in space? But the impacts go a long way. For one it gives the mirage of multiple sources of light as the light path is bent in multiple directions. So one far off star may appear as 2 or even more if there happens to be another celestial body in between the telescope and it. Since it is predicted that there are over 100 billion galaxies on the universe it would not be out of place for there to be a large enough body to affect our views of the universe.
So how does this apply to dark matter? Something we can't even see? Well that's where MACHO's come in. MACHO stands for Massive compact halo object, or in high school terms, a big glob of dark matter. Lets explain, since scientists already know the affects of gravitational lensing they applied that knowledge to examples in the universe where they knew it was occurring. They then found a few strange cases where they knew light was being bent but no celestial bodies were in between the light emitting body and the scientists telescope. SO how could they explain this shift? DARK MATTER! Scientist knew that something was bending the light, and if they couldn't see it, the only explanation they could come up with was the existence of dark matter, a form of matter that is invisible but still has the pull of regular matter.
This discovery and application of science helped scientist find another way to prove the existence of dark matter which is important since it is so hard to prove something that you can't even see!
On my next post I will talk more about evidence of dark matter, this time in the rotational velocity of the galaxies!