Dark Matter

Therefore, astronomers conclude, there must be some invisible matter which produces this extra gravity. Because they cannot see the matter they call it dark matter. One theory is that neutrinos have a little mass, and that this accounts for the dark matter. Another theory is that dark matter could be made of WIMPs.

It seems that this dark matter has existed since the earliest moments of the Universe, so our understanding of the creation of the Universe must remain incomplete until we know what it is made of..