Experiments to detect dark matter, which scientists believe makes up about a quarter of the universe, are underway and may yield direct evidence within a decade
I've never been a fan. I think it more likely that we have gravitation wrong. One possibility is that G has changed over time. Also, I've long wondered what effect instantaneous gravity would have on our model of the universe. Gravitational waves have yet to be directly detected.
EDIT: This just in.