There may be an invisible universe of stars, nebulae, and galaxies composed entirely of dark matter. And astronomers now know how to look for it.

Simply put, dark matter is a mystery. Astronomers have dozens of independent evidence pointing to the existence of a type of nearly invisible matter in the universe. It does not interact with light. However, it exerts a gravitational effect on normal matter. Dark matter holds galaxies together despite their high rotation rates, keeps gas in clusters bound despite its high temperature, bends the path of background light in the universe, and even forms the largest structures in space.

Despite evidence of their existence, the identity of dark matter particles remains unknown. For decades, cosmologists assumed that there was only one type of dark matter particle, a single type that dominated the universe. But recently they began to wonder whether dark matter might be as rich and diverse as the normal universe. For example, some high-energy physics theories predict the existence of a doublet, or mirror, for every particle of normal matter living in the dark sector. In this vision of the universe, there are dark electrons, dark quarks, dark neutrinos, etc., that interact with each other through their own fundamental forces, completely different from the forces we know. It will happen.

This mirror universe may be everywhere but is completely invisible to us. So how can we test this idea? That’s the question a team of astronomers asked in a not-yet-peer-reviewed paper published Nov. 29 in the Preprints Database. arXiv. Surprisingly, they discovered that mirror stars are visible and look completely different from anything we find in the universe.

Mirror stars form when different types of dark matter interact, lose energy, and clump together. This would occur in a process similar to the formation of ordinary stars, in which hydrogen and helium collapse gravitationally, releasing energy through photon emission and becoming dense enough to form stars. However, these mirror stars will interact through their own forces of nature and emit radiation, albeit through the emission of dark photons that we cannot see.

Given that dark matter makes up about 80% of the mass of each galaxy, there may be millions, even trillions, of these dark stars floating around the Milky Way galaxy alone.

But most importantly, as the authors noted, these mirror stars still have gravity. This is how we know dark matter exists. And any large, relatively compact object, whether an ordinary star or a mirror star, will gravitationally attract matter around it. Thus, these mirror stars will attract the gas and dust floating in the interstellar medium.

This simple matter will coalesce on its own into what the authors call “nuggets.” As the nuggets decay, they will heat up and emit radiation. This radiation would appear to be coming from a normal star, but not the kind of star astronomers describe. Instead, the nuggets will be very red because they do not have the warmth of their normal stellar siblings, and they will be very faint because the nuggets are not very large.

But there are other small faint objects in the universe, such as white dwarfs and planetary nebulae. The authors found that they could distinguish these nuggets from white dwarfs based on the wavelengths of the electromagnetic radiation they emit. If we see something that looks like a faint white dwarf but has the wrong spectrum, it could be a chunk of normal matter at the heart of a mirror star. Additionally, these nuggets emit light at wavelengths not found in typical planetary nebulae.

Research shows that although the idea of ​​a mirror universe is highly hypothetical, it is a real, testable scientific idea. If mirror stars exist, there may be nuggets at their hearts, and with sufficiently sensitive and large enough surveys we might be able to find them.