Axions are elusive particles proposed by Roberto D. Peccei and Helen Quinn to explain a puzzle in the Standard Model theory. Frank Wilczek gave them their name. Today they are considered prime candidates for dark matter — an invisible substance that makes up most of the Universe's mass.
Soon after the big bang, тёмная материя was dispersed like vapor. But tiny irregularities acted like dust specks on which vapor condenses into droplets. That's how axion stars formed — invisible spheres of monstrous density.
These droplets of тёмной материи soaked up to half of all the invisible mass in the Universe. That's why detectors on Earth don't see a uniform background of axions — they are hidden inside such stars. However, wandering through magnetic fields of galaxy, these droplets sometimes betray themselves with powerful but short radio bursts. Sensitive telescopes can catch these signals and read the history of the expansion of the universe.
If an axion star the size of a soccer ball landed on Earth, it would weigh as much as Mount Everest but would punch through the planet, barely noticing the resistance.