Do stars made of antimatter exist in the universe? Possibly.
One of the great mysteries of cosmology and astrophysics is that even though equal quantities of matter and antimatter appear to have been produced during the “Big Bang”, today there is only a negligible quantity of antimatter in the observable universe. We do not appear to live in a matter-antimatter symmetric universe.
If antimatter stars, “antistars”, do exist, how could we distinguish them from stars made of normal matter? The light emitted from an antistar would look identical to the light emitted by a normal-matter star.
But if normal matter were infalling upon an antistar, the contact between matter and antimatter would generate an annihilation spectrum of gamma ray photons that peaks around energy 70 MeV (half the mass of a neutral pion) up to a sharp cutoff around 938 MeV (mass of the proton).
A recent analysis of data collected by the Fermi Gamma-ray Space Telescope found fourteen possible antistars. These fourteen point sources produce a gamma-ray signature indicative of matter-antimatter annihilation. These point sources do not exhibit the characteristics of other known gamma-ray sources. For example, they are not, ostensibly, pulsars, active galactic nuclei, or black holes.
The positional error ellipses for these fourteen point sources range from 11×10 arcminutes up to 128×68 arcminutes (95% confidence). Here are optical images of these sources from the Palomar Digital Sky Survey, in order of right ascension (epoch 2000 coordinates).
Since there appears to be no known way to distinguish a star made of antimatter from one made of matter—except for the gamma-ray signature of matter infalling onto the antimatter star, a higher-resolution gamma-ray telescope or interferometer (10 – 1000 MeV) needs to be developed to localize these candidate sources to within a few arcseconds. Higher spectral resolution will help as well, allowing a more detailed characterization of the gamma-ray spectrum.
S. Dupourqué, L. Tibaldo and P. von Ballmoos. Constraints on the antistar fraction in the solar system neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog. Physical Review D. Published online April 20, 2021. doi: 10.1103/PhysRevD.103.083016.
M. Temming (2021, June 5). Antistars could lurk in Milky Way. Science News, 199(10), 8-9.