Most of the universe is not made of the matter that we know of, which comprises the standard model of particle physics. Most of the mass of our galaxy is made of dark matter.
Just what is the evidence for dark matter ?
- Movement of stars in galaxies is too fast, because there is more mass than we can explain.
- Movement of galaxies in galactic clusters is too fast, because there is more mass than we can explain.
- We can see light being bent around the dark matter (called gravitational lensing).
- The formation and evolution of galaxies would be different if there was no dark matter.
- We can observe galactic collisions and see that the mass of the galaxies is not where the matter is.
- The fluctuations of the Cosmic Microwave Background (the afterglow from the Big Bang) are consistent with an additional form of dark matter.
Dark matter can explain all of these observations, and in fact, we have determined the amount of dark matter in the universe to a precision of less than 1%.
So now what ?
Now, we need to figure out exactly what particle dark matter is, and which known or unknown forces it interacts with.