One explanation for all three anomalies is an existing, alternative explanation for the high speeds of stars on the outer edge of the Milky Way, known as Modified Newtonian Dynamics (MOND). Supported by a significant minority of physicists, including Kroupa, MOND assumes that gravity works differently at cosmological scales than it does on the scales we are used to.
Most physicists however think dark matter is still too successful to abandon, and that MOND is too immature to act as a replacement. "MOND has done an incredible job explaining rotation curves really accurately," Smith says. "But at how well it's been tested, at the moment it's not in a place to replace dark matter."
References: Moni Bidin et al: arxiv.org/abs/1204.3924 (Astrophysical Journal, in press); Karachentsev: arxiv.org/abs/1204.3377 (Astrophysical Bulletin, in press); Kroupa: arxiv.org/abs/1204.2546 (Publications of the Astronomical Society of Australia, in press)
The ongoing WIMP war
Even for dark matter's biggest fans, there's trouble brewing. Labs across the world are waiting with detectors wide open for hypothetical dark matter particles called WIMPs (weakly interacting massive particles), but what they are seeing is not easy to explain.
The "WIMP wars" have raged since 1998, when the DAMA experiment in the Gran Sasso lab in Italy claimed its detector was sparkling with particles that could be WIMPs. The team reported the same in 2008, but no other experiment had seen anything.
The plot thickened in 2011, when two other experiments - CRESST II, also in Gran Sasso, and CoGeNT, housed in the Soudan mine in Minnesota - reported flickers of dark matter in their detectors, too.
That would have made a pretty strong case, if not for two similar experiments that have seen no dark matter at all. Xenon100, a tub of liquid xenon in Gran Sasso, and CDMS II, next door to CoGeNT, have so far come up empty.
CDMS presented its most recent disappointing results in February at the Dark Matter 2012 conference in Los Angeles. Subsequently, Juan Collar, a member of the rival CoGeNT team at the University of Chicago noticed that their results only considered WIMPs above a certain energy. When he re-analysed the CoGeNT data for WIMPs of lower energies too, he found a signal that might be consistent with particles of dark matter (arxiv.org/abs/1204.3559).
Sadly, the signal still differs from the detections by the other groups, so Collar is not announcing evidence for dark matter yet. "I'm personally not ready to believe that we're seeing WIMPs," he says.
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