Dark matter is a mysterious phenomenon. We do not know what it looks like or what it is made of. But physicists are convinced that it exists, because the effect of its gravitational pull can be observed in many examples in the cosmos. The visible universe – ourselves, the planets and stars – accounts for 25 percent of the total mass in the universe. The remaining 75 percent of its mass consists of dark matter.

The fact that it interacts via gravity at least gives researchers a clue as to how heavy its particles might be. It was previously assumed that their mass would have to be between 10^-24 eV and 10²⁸ eV. That’s a huge range, spanning 52 orders of magnitude, starting at the fraction of a neutrino and ending well above all known particles. Researchers led by Professor Xavier Calmet of the School of Mathematical and Physical Sciences at the University of Sussex have now succeeded in providing a more accurate estimate of this.

Their results, published in March in the journal Physics Letters B, radically narrow the range of possible masses for dark matter particles and help focus the search for dark matter in the future. The University of Sussex researchers used the fact that gravity acts on dark matter in the same way it does on the visible universe to calculate the lower and upper limits on dark matter mass.

The results show that dark matter cannot be «ultralight» or «superheavy,» as some theories claim, unless a yet-undiscovered force is also acting on it. The team assumed that the only force acting on dark matter is gravity, and calculated that dark matter particles must have a mass between 10^-3 eV and 10⁷ eV. That’s down to ten orders of magnitude.

What makes the discovery even more significant is that if it turns out that the mass of dark matter does lie outside the range predicted by the Sussex team, then this also proves that an additional force – besides gravity – must be acting on dark matter.