Dark matter has turned out to be more elusive than physicists had hoped
We can measure the gravitational pull of this “dark matter” on the orbits of stars and galaxies. We can see the way it bends light around itself and can detect its effect on the light left over from the primordial plasma of the hot big bang. We have measured these signals with exquisite precision. We have every reason to believedark matter in experiments for decades now, to no avail. Maybe our first detection is just around the corner.
The changes would have to mimic the effects of dark matter in astrophysical systems ranging from giant clusters of galaxies to the Milky Way’s smallest satellite galaxies. In other words, they would need to apply across an enormous range of scales in distance and time, without contradicting the host of other precise measurements we’ve gathered about how gravity works.
WIMPs appeal to many experimentalists because they must interact significantly with the known particles—that’s how they arrive at the right amount of dark matter.
Because of how weakly QCD axions are expected to interact with regular matter, fewer experiments have looked for them, and they have searched in only a tiny fraction of the possible mass range. New detection strategies and quantum sensor technologies, however, have opened up prospects for hunting the QCD axion over many orders of magnitude in mass.
What about the other end of the mass scale? The smallest clumps of dark matter we can directly observe are tens of millions of times the mass of the sun. Individual dark matter particles should be smaller than that, but how much smaller? If dark matter were made of dense, dark objects—often called massive compact halo objects , as a tongue-in-cheek contrast to WIMPs—then their gravity could deflect light and disrupt orbits as they barreled through the galaxy in ways we could see.
The only way we know to search over such a wide range of possibilities is to build many small experiments, each sensitive to different types of dark matter, rather than focusing our resources on a few huge projects. We can also use these small experiments to develop new technologies and try out novel ideas; if one of those strategies proves powerful or detects something that could be an initial hint of dark matter, we could then scale it up.
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Dark Energy Camera's new galactic portrait delves into dark-matter centralKeith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester.
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