There is clear evidence that matter “that neither emits nor absorbs electromagnetic radiation” exists, so why haven’t decades of search turned it up*?
Mario Livio and Joe Silk tell us at Nature,
We know a little about dark matter. Because it does not absorb light or interact with electromagnetic waves, the majority of it cannot be made of baryons — particles of ordinary matter, including protons and neutrons, which are composed of three quarks. And dark matter must lie beyond the standard model of particle physics to avoid upsetting Big Bang nucleosynthesis, the theory of which successfully predicts the observed abundances of light elements such as deuterium, helium and lithium arising from interactions in the early Universe.
The main constituents are expected to be weakly interacting massive particles (WIMPs). These particles have masses a few tens to thousands of times that of the proton. WIMPs interact among themselves and with ordinary matter gravitationally and through the weak force, but not electromagnetically or through the strong nuclear force. To explain the way in which galaxies form and cluster, dark-matter particles should be relatively slow moving, or ‘cold’. If they were faster, and could move easily beyond the dimensions of a protogalaxy, many structures visible today would have been washed out.
In short, as befits its name (though given for other reasons), what w know is mainly what it can’t be.
Livio and Silk suggest ramping up the energy of current detectors such as the Large Hadron Collider, scheduled to resume in 2015.
But there is another possibility:
Some theorists have even started to wonder whether dark matter exists. Since the 1980s, a few have proposed modifying the theory of general relativity to do away with the need for dark matter. Such radical ideas are increasingly invoked to address another grave problem in astrophysics: the origin of the ‘dark energy’ that accelerates the expansion of the Universe. Most researchers think that we are far from needing new physical laws, especially because experimental avenues are still open. But unpleasant surprises are always possible.
There are two worst-case scenarios. First, dark matter may not comprise one type of particle — as many current searches assume — but many. Second, the particles might interact only gravitationally, and could be practically invisible to conventional detectors.
So in fifty years, the dark matter field could be where medical diagnostics are (astonishing) or where origin of life is (glum).
*Of course we could ask the same question about the space aliens, but in that case, the answer that they don’t exist is quite reasonable. There was never compelling, publicly available evidence.
See also: The Science Fictions series at your fingertips (cosmology).
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