According to most physicist and astronomies the dark matter particle has yet to be found. At yet as early as 2001, Scientists at the DAMA experiment at Gran Sasso had detected a signal of the earth moving past streams of dark matter particles in the galaxy. As the years went on the signal got stronger and stronger. Until mid-way through last month, DAMA announced detection of an annual modulation signal with a confidence level of 8.2 sigma. In science a confidence level of 5 sigma is normally enough to mark a discovery, and since the confidence scales exponentially, 8 sigma is very clear and loud signal indeed.
So are scientist now, proclaiming dark matter as found. No, in fact the reaction has been extremely critical. For example, Juan I Collar, himself a builder on DM detectors, writes critically at Cosmic variance:
There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right?
Well that pretty negative, especially as he didn't advance any theory as to what the DAMA signal was. One of the problems with physicists believing in the DAMA signal is that it doesn't match with the negative results from all the other detectors so far. In fact combining the DAMA signal the the negative results from CDMS II pretty much rules out the standard super-symmetric WIMP candidate, the neutralino. The only theory advanced so far, that is compatible with both experiments is Mirror Matter. As described by Robert Foothere. In fact this as an updated version, of his similar paper, based on the older less accurate results from DAMA and CDMS four years ago. As the measurements get more accurate, the evidence for the Mirror Matter dark matter become stronger and for super-symmetricdark matter weaker. Not that most physicists are playing much attention. Or that experiments specifically tuned for mirror matter are being made.
This is a real pity, its not like mirror matter is cranky, in its own way, its far more believable than, say, String theory. It just a second copy of the standard model, with identical particles, except swapping left and right handedness. Mirror Matter, gets its own mirror forces, and mirror light. Only gravity is shared between the two. Also maybe a tiny, about billionth part of the electric forces mix with the mirror electric force, to allow a tiny interaction between mirror particles and ordinary one. I hope the new results will prompt the physics community to put a bit more effort in the mirror matter.
BTW, does the DAMA experiment, mean anything for my pet, axial force theory. Actually, no. The axial force theory has very little to say about dark matter, it only describes dark energy. The only difference the axial force makes to dark matter theory, is that if there is an axial force, and you make a super-symmetric theory with it, you don't need R-parity (an ad'hoc symmetry, where known particle have R=1, and there super-symmertic partners, have R=-1), in order to make the proton stable. Conservation of the electric and axial charges is enough to make the proton exactly stable. No R-parity means, the neutralino probably isn't stable, and so isn't a good WIMP candidate any more. Discovery of a stable neutralino, would be bad news for the axial force. Is this why, I'm touting mirror matter theory. Actually no, mirror matter appealed to my physicists intuition, long before i considered an Axial force.