The vast majority of our universe should be made up of dark matter that is invisible to our instruments, not to mention our senses. The reason is that dark matter should be very inert to ordinary matter and radiation. No wonder scientists are still escaping and hiding their identity from them. We know, or rather suspect, its existence due to the gravity that dark matter exerts on the surrounding universe.
Nevertheless, there are some ways in which we can detect dark matter with existing devices. If certain conditions are met. For example, if the dark matter were formed by some ultralight axion-type particles, such particles could rarely interact with each other, leading to their annihilation. If the energy of such collisions is sufficient, they could produce gamma photons.
The gamma photon would then decay into an electron and its antimatter counterpart positron. Electrons with positrons could together form exotic “atoms” called positronium. They are not stable and would soon disintegrate, with an accompanying flash of radio radiation. Although it would still be true that we cannot detect dark matter as such, it is something else with the mentioned radio radiation.
For it to work and for us to observe, it must be a large amount of dark matter. In our galactic neighborhood, the density of dark matter is probably much lower than would be necessary for such an observation. The center of the galaxy is a much better choice.
Will research by globular clusters help?
However, the center of our Milky Way is not the most suitable for this purpose. It is literally permeated by all kinds of radio emissions and it would be extremely difficult to distinguish a possible radio signal from them by annihilation of dark matter particles.
Australian astronomer Lister Staveley-Smith of the International Center for Radio Astronomy Research (ICRAR) and the University of Western Australia and colleagues suggest using other, smaller, but still quite dense clusters of dark matter, which experts believe are found in centers of globular clusters, which in many cases are former dwarf galaxies.
Staveley-Smith et al. apparently bets that the dark matter is made up of said ultralight particles. They explored two nearby globular clusters, 47 Tucanae and Omega Centauri, which are relatively easy to observe. They found no radio signals from positron decay, but that’s just the beginning. They intend to continue the search for radio tracks in the dark.
Cover illustration photo: NASA Universe, CC BY 2.0