From the BBC:
What is our Universe made of?
Billions of dark matter particles pass through us every second. “They are in your office, in your room, everywhere,” says Frenk. “They are crossing through your bodies at a rate of billions per second and you feel nothing.”
There have been some false alarms along the way
In theory we should be able to spot the little flashes of gamma rays from these collisions. The trouble is, lots of other things are also passing through, including radiation in the form of cosmic rays, and this swamps the signal from the dark matter.
Hence the underground experiments: the rocks above block most radiation, but allow dark matter through.
So far, most physicists agree we have not yet seen any convincing signals from these detectors. A paper published in August 2015 explains that the XENON100 detector in Italy’s Gran Sasso National Laboratory has failed to find anything.
It’s been over 80 years since Zwicky first suggested the existence of dark matter. In all that time, we haven’t been able to get hold of a sample, or nail down what it is.
It’s a humbling reminder of how far we still have to go before we really understand our Universe. We may understand all sorts of things, from the beginning of the Universe to the evolution of life on Earth. But most of our Universe is still a black box, its secrets waiting to be unlocked.
The Case for Complex Dark Matter
Bullock thinks that dark matter might instead be complex, something that interacts with itself strongly in the way that ordinary matter interacts with itself to form intricate structures like atoms and atomic elements. Such a self-interacting dark matter, Bullock suspects, could exist in a “dark sector,” somewhat parallel to our own light sector, but detectable only through the way it affects gravity.
He and his colleagues have created numerical simulations that predict what the universe would look like if dark matter feels strong interactions. They expected to see the model fail. Instead, they found that it was consistent with what astronomers observe.
Quanta Magazine spoke with Bullock about complex dark matter, how this mysterious mass might behave, and the best places in the universe to find it. An edited and condensed version of the interview follows.
We’ve known that there’s a bit of a problem at the centers of galaxies for about 20 years. At first it was thought maybe we’re interpreting the data wrong. And now the question comes down to: Does galaxy formation eject dark matter somehow, or do we need to modify our understanding of dark matter? More.
From Real Clear Science:
The Search for Dark Matter Just Got Interesting
The new XENON100 report has found no evidence of WIMPs scattering off electrons. Although this is a negative result, it rules out many so-called “leptophilic” models that predict frequent interactions between dark matter and electrons.
But the most important consequence of the XENON100 analysis is with regards to the controversial claim of dark matter detection by researchers at the DAMA/LIBRA experiment in Italy, which is in conflict with the results from many other detectors such as the Cryogenic Dark Matter Search. Leptophilic dark matter was proposed as a viable explanation for this discrepancy since exclusions from other experiments would not directly apply. However, the new results from XENON100 firmly rule out this possibility. More.
Our friendly local physicist Rob Sheldon notes,
Just for the record, the article referenced reports on two more excluded theories of dark matter, where several $20M experiments revealed nothing.
The total number of failed WIMP experiments is now > 50, and their combined budget is close to $1bn. If we include astronomical searches for WIMPS, then we have something like >150 failed experiments. On the other hand, the MACHO search had a few astronomical collaborations back in the 90’s, and recent proposals have been denied funding because WIMPS rule, or something. Strange how a scientific consensus can form around known-to-be complete failures.
Yes but, Rob, getting people used to failure is an important part of modern cosmology. We’ll be seeing plenty of it. We can’t afford success.
See also: But why is the quantum world thought spooky anyway
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