Neutralinos aren’t the only kind of dark matter that scientists have suggested could form stars. Some researchers think there could still be dark matter stars out there made from bosons. Boson particles carry the fundamental forces, including the strong, weak, and electromagnetic force. For example, photons are bosons that carry the electromagnetic force.
But some scientists think that dark matter could have its own boson. These dark bosons could be dark matter itself, or they could simply cause its interactions with regular matter. Dark matter bosons also might be able to coalesce into dark matter stars, though they’d behave rather strangely.
Ordinary stars like our Sun are made of fermions, such as protons, electrons, and neutrons. But boson stars would be made of bosons, which follow different rules than we're used to in our day-to-day experiences.
For example, the fermions in normal stars behave like people. Two people can’t occupy the same physical space at the same time, and they have to take turns standing in a given spot. But bosons act more like waves in water. And when waves come together, they don't push each other out of the way. Instead, they add together and make a bigger wave. In physics, this is called a Bose-Einstein condensate. A dark boson star would be like a giant, dense Bose-Einstein condensate in outer space. And it might resemble another extremely compact object — a black hole.
In fact, a recent study published in the journal Physical Review Letters suggested that scientists may have already witnessed two boson stars collide. In May 2019, the Laser Interferometer Gravitational-Wave Observatory caught the ripples in space-time caused by the collision of two weirdly large black holes. The pair were some 66 and 85 times the mass of our Sun, respectively, which is larger than current theories suggest should be possible through the death of massive stars.
It’s possible that each of these black holes were the products of previous, smaller black hole mergers. But after the team ran that option through their models, they realized that such an outcome would be indistinguishable from the collision of two dark boson stars.
If it turns out the universe is replete with these hypothetical dark boson stars, it would finally explain the enduring mystery of why scientists haven't been able to track down dark matter.
Quark stars