NASA has announced yesterday that it has found direct evidence of dark matter in the collision of two large galaxy clusters. This was possible because dark matter does not interact with "normal" matter or with itself in any way other than by gravity, while normal matter is also affected by friction; because of that, "normal" and dark matter were separated during the collision and the gravity-lensing effects of both could be independently measured.
This gives a serious push to the theory that most of the matter of the universe is actually dark matter; the theory explains many observations that would not be compatible with the way we understand gravity to work if only "normal" matter were involved. Other competing theories exist, but none of them explains the observed effects from the galactic collision observed by the Chandra X-ray Observatory. That's not to say that none will show up in the future, but this does add one more hurdle to competing theories.