So, over the years there's first been an intensive hunt, and then more and more indirect to direct evidence of dark matter, which is stuff that isn't like ordinary matter and was postulated long time ago because the Theory of Relativity predicts rotational motion of galaxies to only be possible if far more matter than just visible matter exists.
Of course, there were alternative theories, but over time it seems the theory of Relativity wasn't the culprit and instead the universe is mostly made up of dark energy and dark matter (no direct evidence for dark energy yet though).
So, now it seems we have the most conclusive proof dark matter exists:
"Astronomers using the Hubble Space Telescope spotted a ring of dark matter in a galaxy cluster some five billion light-years away from Earth, which measured some 2.6 million light-years across.
"This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster," said James Jee, a member of NASA's team of astronomers.
Astronomers have long suspected there must be some substance holding galaxy clusters together, otherwise galaxies would only have the gravity from their visible stars, which would not be enough to keep them from flying apart.
Though invisible, astronomers have inferred dark matter exists by observing how its gravity bends the light of more distant background galaxies.
"Although the invisible matter has been found before in other galaxy clusters, it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters," said Jee, who works at the Johns Hopkins University."