How much do dark matter particles weigh? Premium

Dark matter's mass limit increased by an order of magnitude, impacting our understanding of the universe's invisible substance.

Dark matter is an enigmatic invisible substance supplying five-sixths of the matter of the universe. Unlike photons, the particles of light, the particles of dark matter need to have non-zero mass or else the dense and intricate structure of matter on cosmic scales will not form.

How light can a dark particle then be? For decades scientists thought this minimum mass was about 10-31 times the mass of a proton. But in May this year, theoretical physicists revised the limit and pushed it up by an order of magnitude, to 2.3 × 10-30 proton masses. This is a significant update in the world of dark matter.

To understand these numbers and their importance, let us first build a mental picture of dark matter. Dark matter is said to be everywhere in the universe. Does that mean it is in your house? In 1922, Dutch astronomer Jacobus Kapteyn studied the motion of stars neighbouring the Sun and concluded the density of “dark matter” (using that term for one of the first times) must be 0.0003 solar masses per cubic light year.

Since then, through a century of increasingly sophisticated measurements, the accuracy of Kapteyn’s conclusion has held up remarkably well. This density of dark matter can be re-expressed as the heft of two protons per teaspoon, which means your house could contain dark matter with a mass equivalent of a trillion protons.

But this would also be naïve: Kapteyn’s and subsequent measurements are only valid when regarding the million-cubic-lightyear volume and doesn’t apply when we zoom in for a closer look. This is because stars, whose motion is used for the measurement, are themselves separated by a few light years. Whether or not dark matter is present on smaller length scales would depend on how it is distributed: either uniformly or in lumps.

Let’s assume it is spread around like fine flour, which the standard theories of cosmology also predict. If it comes in lumps, the spacing between them may be as large as many light years and there will perhaps be no dark matter under your roof.

Now, since we know the local density of dark matter, the value of the unknown mass of the dark particle will determine the separation between two neighbouring particles. If it is 100 proton masses, the inter-particle separation will be 7 cm. Then dark particles at any given moment will not only be in your house but also in your head.