Possible Dark Matter Detected

Dark matter, as you might gather from the name, is impossible to "see" (in the sense of reflecting light).  Whatever it may be specifically, it most certainly doesn't seem to interact electromagnetically with any other particles.  And so we turn to other forces to detect it - most notably gravity.  It's existence was first inferred from the inexplicably high gravitational energy necessary for the observed speeds of galactic rotations (and the speeds of galaxies in clusters).

"Hang on, Doc.  I see something that makes no sense."

In fact, since it doesn't interact with light matter (electrons, nuclei, all that 'stuff' you're so familiar with), but still has an assumed mass, one would expect it to bend spacetime, and maybe when galaxies collide, the dark matter would drift out from them, unimpeded.  So we found that too:

The light matter is shown in red (from emitted X-Rays)
The lensing effect of dark matter shown in blue.

It is also suspected that dark matter particles interact with the weak force, and so are named Weakly-Interacting Massive Particles (yes, WIMPs) - not to be confused with much less massive particles with also interact with the weak field but not electromagnetic fields, such as neutrinos.  This can be fairly difficult, as the weak force operates over a very short range (on the scale of an atom), and so the particles need to get very close to an atom's nucleus to have an effect.

And now we may have detected actual, individual dark matter particles.  Three of them, to be precise:

Eight silicon detectors recorded three events that may represent collisions from weakly interacting massive particles, or WIMPs. Physicists have found hints of the existence of WIMPs before, but they remain elusive. Two other possible detections from the CDMS search, reported in 2010, turned out to be indistinguishable from background collisions from other, non-WIMP, sources. The same may yet hold true for the latest findings.

The work was reported on 13 April at the American Physical Society meeting in Denver, Colorado, and is published at arXiv.org. “We do not believe this result rises to the level of a discovery, but it does call for further investigation,” said Kevin McCarthy, a CDMS team member from the Massachusetts Institute of Technology in Cambridge.

 They're over 90% certain it's an unaccounted-for particle (when you consider all the background noise and effects of known particles), which might be an acceptable risk if you like gambling, but physicists prefer to be very confident before they report data as an actual "discovery" (recall the discovery of the Higgs boson, which they weren't comfortable claiming until they were over 99.9999% certain).

Hopefully "further investigation" yields supporting results.  One way or another, we are definitely closing in on this invisible substance that makes up nearly all of the mass in the universe.