By measuring the motion of stars in our galaxy and others, astronomers can tell that galaxies in general contain much more mass than can be accounted for by their visible stars, gas and dust. In fact, normal, or baryonic, matter (essentially anything with protons and neutrons) seems to account for just 15 per cent of all the mass in the universe.
The rest is composed of something else – something that’s not just dark, but entirely transparent and unaffected by any kind of radiation. Dark matter plays an important role in the structure of the cosmos too – its enormous gravity causes baryonic matter to cluster around it, coalescing into galaxies and galaxy clusters. As a result, its distribution is similar to that of visible objects.
As for what exactly it is, current research points to some kind of undiscovered heavyweight particle, capable of passing through baryonic matter as if it weren’t there. Astronomers and physicists have made attempts to detect these weakly interacting massive particles (or WIMPs) and measure their properties, but so far they’ve eluded them.
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