No WIMPs

What is Dark Matter? We can see its gravitational effects, but we have no idea what it is. One candidate has been Weakly Interacting Massive Particles (WIMPs), that is, particles of ordinary matter that for some reason don't interact with other types we are more familiar with. According to this theory, trillions upon trillions of these ghostly particles might be passing through every second without your feeling a thing. But if our physics is any good such particles should leave a trace when they smack right into a quark, and given how many WIMPs there would have to be to account for all the extra gravity this ought to happen on a regular basis. So scientists have been trying to find evidence of  such collisions. But they have not:

The latest, most sensitive search for particles of dark matter—the bizarre invisible stuff in which our galaxy appears to be embedded—has come up empty. Since 2012, physicists working with the Large Underground Xenon (LUX) detector had been searching for evidence of so-called weakly interacting massive particles, or WIMPs, bumping into the atomic nuclei in 370 kilograms of frigid liquid xenon. But the experiment, which is housed 1480 meters deep in the Sanford Underground Research Facility in Lead, South Dakota, ended its final 20-month run in May, and researchers see no evidence for such particles.

The search goes on.