To understand dark matter, we need to establish what ordinary matter is. Matter includes everything you can touch, such as bananas, iPods, cars, aircraft carriers or people; everything you could ever possibly touch, for example, the Moon, Mars, aliens and even asteroids on the other side of the Universe; and everything you could ever theoretically touch, such as the surface of the Sun. So in short: everything you see, including everything you see in the night sky, is made up of matter.
Every single piece of matter, regardless of its size, exerts a small attractive force called gravity. Gravity is what causes the Earth to orbit the Sun and is what astronomers use to detect planets around distant stars. Now just as the Earth orbits the Sun, the Sun orbits the centre of the Milky Way. That is why when observing distant galaxies astronomers can calculate how much matter there is by how fast the galaxy is rotating.
The first evidence of dark matter
In 1932 a Dutch astronomer named Jan Oort (1900-1992) realised that the galaxies he was observing were spinning way too fast for the observed mass to allow. Other astronomers have also observed countless galaxies and galaxy clusters spinning too fast for their observed mass to allow, and they have concluded that there must be a whole lot of matter in the galaxies that they can’t see. That invisible matter is what we now know as dark matter.
Astronomers detect objects in space by taking images of the sky. They use radio waves, which are the ones you listen to on the car radio; microwaves, which you use to cook dinner in a microwave oven; infra-red light, the one you see in thermal imaging; ultraviolet light, what causes sunburn; X-rays , what we use to see broken bones, and gamma rays, what’s released from nuclear bombs. But none of these techniques allows them to actually see dark matter, because it doesn’t emit or reflect anything!
The most mind-blowing thing, however, is the amount of dark matter that exists out there in the universe: astronomers estimate that there is about five times more dark matter than ordinary matter, which means our universe is comprised of more than 24% dark matter.
The scientific community is still trying to figure what dark matter is made up. So far they think that it is composed of exotic particles that were formed when the universe was just a few light-years old.
Check out this phd comics video that delves into dark matter, our universe and gravity: http://vimeo.com/22956103
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