So, What Is Dark Matter?
“Dark matter has been a polarizing subject. It hasn’t been detected, the name implied it was a mystery, and it started out as an explanation for the apparent extra but invisible mass in galaxies.
But the evidence that something unknown is out there has become rather encompassing, appearing in the cosmic microwave background, galaxy clusters, and even apparently empty space. Even if dark matter doesn’t exist, something will have to fill a whole bunch of gaps at many different scales of the Universe.
Nevertheless, it is a placeholder concept, a hole in our knowledge that we can feel the shape of but haven’t yet managed to capture in the spotlight.
So, what is dark matter?
One possible answer is a modified theory of gravity, but the favorite proposal at the moment is a class of particles called weakly interacting massive particles (WIMPs). The distinguishing feature of WIMPs is that they are not dark at all—instead, they interact so rarely with normal matter that our current instrumentation is blind to their effects.
The fact that these particles interact at all is probably one of the main reasons that physicists prefer the WIMP explanation: if WIMPs exist, we could build an instrument to see them.
And build them we have. Two teams have now claimed to have detected dark matter particles. But last month the XENON100 team published its own data, claiming that the earlier results are bunk and dark matter cannot possibly have been detected.”