Perhaps the greatest problem in astrophysics is the mystery of dark matter. When astronomers observe other galaxies, they can see how fast these objects rotate. They can also see how much visible mass they contain and calculate the gravitational forces it must generate.

And therein lies the problem. The visible matter simply does not generate enough gravity to hold the galaxies together. By this measure, they ought to fly apart.

So astrophysicists believe that something else must be holding the galaxies together. Enter dark matter—some kind of mysterious, invisible stuff that generates the gravity that holds the universe together.

The big question is: what is this stuff? Today, Vyacheslav Dokuchaev and Yury Eroshenko Institute for Nuclear Research of the Russian Academy of Sciences in Moscow propose an interesting new idea.

These guys say that dark matter is composed of black hole atoms—microscopic black holes with a charge that have somehow captured an electron or proton leaving them electrically neutral. “We propose these black hole atoms as the possible origin of dark matter particles,” they say.

Physicists have long thought that microscopic black holes must have formed in the early universe. That’s because quantum fluctuation in the density of matter at this time would have created some regions of space that were dense enough to form black holes.

Some of these would have been huge, perhaps seeding the formation of galaxies. But most would have been much smaller—microscopic, in fact.

The thinking is that today, the universe must be filled with these objects. Various theorists have discussed the possibility that these so-called ‘primordial black holes’ could be the mysterious dark matter.

One problem is the possibility that primordial black holes may have an electric charge. In that case, it’s quite possible that they would have attracted protons or electrons, leaving them electrically neutral, just like atoms.

The question that Dokuchaev and Eroshenko address is what properties such objects would have. And the answer is exactly the properties you’d expect of dark matter.

For a start, black hole atoms would be massive, from 10^14 kilograms to 10^23 kilograms, about the same mass range as asteroids. But even at that mass, they would be tiny: smaller than atoms but larger than nucleons, such as protons and neutrons.

They would also be unlike any other form of matter in the universe. Dokuchaev and Eroshenko calculate, for example, that an electron (or proton) could orbit inside the black hole’s horizon, meaning that it can never escape.

That would also limit the interaction with ordinary matter. So despite their mass, black hole atoms would interact with other matter less strongly than neutrinos. “The interaction of the neutral black hole atoms with ordinary matter via the gravitational dynamical friction effect is extremely weak,” they say.

The bottom line is that black hole atoms are dark, massive, non-interacting particles. “These properties are just one needs for the dark matter candidates,” say Dokuchaev and Eroshenko.

So how to detect them? Not very easily. Dokuchaev and Eroshenko say that the formation of these atoms might produce a measurable signal. When an electron tunnels into a microscopic black hole to form a black hole atom, it ought to release energy in the form of a flash of ultrahigh energy cosmic rays.

What’s more, the electrons may be quantised, just as in ordinary atoms, and the jumps from one level to another would release photons. “This effect makes “black hole atoms” observable in principle,” they say.

That’s an interesting idea. But it has its fair share of problems too.

If black hole atoms are dark matter, then they must fill the universe. Perhaps the most serious problem is finding a way to unambiguously observe these objects and distinguish them from ordinary black holes that are electrically neutral or indeed other proposed candidates for dark matter.

But there are also exciting possibilities too—that black hole atoms might interact with each other in a way that forms molecules and other larger things, for example. Just what structures might form is an open question.

There’s more work here for black hole theorists, should they have a few hours to spare.

Ref: arxiv.org/abs/1403.1375 : Black Hole Atom as A Dark Matter Particle Candidate