14 DAY TRIAL // After a large number of scientific experiments failed to discover any signs of dark matter, scientists are beginning to reexamine all proposed dark matter candidates, or particles that make up this proposed type of matter. Until now, the search has been focused on weakly-interacting massive particles (WIMP).
Dark matter was proposed as a consequence of Albert Einstein's theory of general relativity, and is believed to make up nearly a quarter of the Universe's mass-energy budget. However, it is invisible to regular detection method, since it does not release light.
Its effects on regular, baryonic matter can only be measured based on how the two interact via gravity. What astrophysicists do know is that dark matter tends to concentrate at the cores of galaxies and galaxy clusters, and to form filaments between these large-scale structures.
One of the leading candidates for this elusive type of matter is WIMP, which scientists say meets all predictions related to dark matter. However, searches and experiments meant to identify signs of WIMP have thus far come up empty-handed, forcing experts to reconsider candidates that were previously ignored.
A significant concern that theoretical physicists have is that dark matter may turn out to be made of particles that are virtually undetectable, regardless of how sensitive our detectors are. An important thing to note here is that no substantial proof exists that dark matter is real at all.
Scientists received a new blow late last year, when the results of the most sensitive dark matter detector in the world, the Large Underground Xenon (LUX) project in South Dakota, revealed no trace of WIMP after more than three months of continuous analysis.
If these particles are indeed responsible for making up dark matter, then they are running out of places to hide. LUX scientists say that they have already shown half of existing WIMP models to be false, and add that it is only a matter of time until they go through the remaining ones as well.
Axions are another popular particle candidate for dark matter. They are believed to be much smaller than their weakly-interacting counterparts, but the downside is that theoretical models indicate a much smaller chance of them interacting with regular matter as well, Nature reports.
“I don’t understand why axions tend to get ranked as number two. I would invert the order. But that’s my opinion,” comments University of Washington physicist Leslie Rosenberg, the leader of the Axion Dark Matter eXperiment (ADMX)
Besides WIMP and axions, there are some other models to explain dark matter. For example, one idea holds that a large number of black holes scattered throughout the Universe may be responsible for producing the effects we now attribute to this elusive form of matter.
Yet another possibility is that dark matter may in fact be quark matter, and consist of strange quarks in an extremely dense phase of matter. These quarks should in theory be produced by neutron stars, and may in turn form quark stars capable of exerting massive gravitational pulls on their surroundings.
“I doubt we’ve thought through all the interesting possibilities. We may get lucky, or this may drag on for 100 years or more,” says Harvard University visiting theoretical physicist Matt Strassler.
The worst-case scenario is one where dark matter is made out of particles that simply do not interact with baryonic matter through anything else other than the force of gravity. If that is the case, then there is simply no way for a dark matter detector to find signs of it, no matter how sensitive it is.
“If we move into a mode where our most favored particles are simply not detectable, we have the classic scientific challenge, which is how do you verify such a theory? At that point you’re almost a failure – you have a theory that’s almost impossible to test,” concludes LUX co-spokesman Richard Gaitskell, a professor at the Brown University.