August 30, 2016 | David F. Coppedge

Dark Matter: Now You See It, Now You Don't

All detectors fail to find dark matter. So how can a whole galaxy be made of it?

PhysOrg announced, “Scientists discover a ‘dark’ Milky Way: Massive galaxy consists almost entirely of dark matter.” How can they tell? A study of the motions of a galaxy nicknamed Dragonfly 44 shows that “It has so few stars that it would quickly be ripped apart unless something was holding it together.” That something must be Dark Matter.

Trouble is, when they go looking for that something, they can’t detect it.

The Dark Matter is not MACHO. PhysOrg says that a star cluster has ruled out the hypothesis that dark matter is made of “Massive Compact Halo Objects” (MACHOs, to counteract another theory that dark matter particles are WIMPs—Weakly Interacting Massive Particles).

The Dark Matter is not sterile (sterile neutrinos, that is). writes, “Still No Sign of ‘Sterile Neutrino’ Particle, Candidate for Dark Matter.” The IceCube detector in Antarctica fails to find that candidate, even in this election year. New Scientist says the detector “rules out” sterile neutrinos.

Can one enigma solve another? Science Daily says that astronomers at Johns Hopkins are doing that. They are studying mysterious fast radio bursts as possible probes into the enigma of dark matter. No results yet.

Dark matter is not decaying like it should, Nature News says. That is, if dark matter even exists. “A survey of X-ray light from galaxy clusters has found no evidence of dark matter decaying, in the latest in a series of contradictory results.” Headline: ‘Dark-matter evidence weakens.’

The candidate pool is shrinking. Sarah Lewin at New Scientist reports on negative findings from the orbiting Fermi Gamma-Ray Space Telescope. All the candidates – WIMPs, MACHOs, neutrinos, axions – have left the campaign or are dropping in the polls.

Some astronomers are looking for clues by looking at nothing. reports on astronomers who are studying the empty space between bright things, the voids that might give clues to the dark mysteries of cosmology. PhysOrg calls this ‘much ado about nothing’ – “Astronomers use empty space to study the universe.” By staring at what’s not there, they hope to learn about another thing that must be there but is not: dark energy.

Antimatter Problem

Neutrinos are coming in handy, on the other hand, to help solve another cosmological mystery: where is the antimatter? There should be an equal amount of matter and antimatter in our universe, but antimatter is extremely rare.

Astrophysicists’ strategy to explain this is to find some tiny, tiny difference between particles of one or the other, such that after they annihilated each other, a fraction remained to become our universe. New Scientist teases with a glimpse of a start of a beginning of a possible theory:

It could all have been so different. When matter first formed in the universe, our current theories suggest that it should have been accompanied by an equal amount of antimatter – a conclusion we know must be wrong, because we wouldn’t be here if it were true. Now the latest results from a pair of experiments designed to study the behaviour of neutrinos – particles that barely interact with the rest of the universe – could mean we’re starting to understand why.

PhysOrg hypes the hope with, “Evidence mounts that neutrinos are the key to the universe’s existence.” Science Daily warns that this is “by no means a definitive discovery.” Nature says that excitement is rising over the possibility that morphing neutrinos might address this decades-long puzzle. So far results have just constrained a candidate theory, another Nature News piece cautions.

Monopole Problem

Followers of cosmology may know about another big-bang mystery, the missing magnetic monopoles. In an Explainer at The Conversation, T’Mir Danger Julius from Swinburne University of Technology explains the mystery and what monopoles should be, if they exist.

Do you get the feeling that cosmologists are obsessed at looking for ghosts? Where do they get the money? Their prize would be to announce that their consensus theory finally has some support. Too bad dark matter is not the only falsification.

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