When a massive star explodes as a supernova, a super-dense sphere remains — a neutron star. It's the size of a city but weighs more than the Sun. If the remnant were just a bit heavier, it would become a black hole. Around such stars, mysterious dark matter can gather — invisible substance that, according to calculations by astronomer Vera Rubin, emerged after the Big Bang and fills the cosmos.

Imagine a snowball rolling through different powders. Light powder clings to the outside, making it fluffy and large. Heavy metal balls, on the other hand, sink to the center, forming a hard core. That's roughly how dark matter behaves inside a neutron star. Lightweight particles create a diffuse halo, puffing up the star, while heavier ones accumulate into a tiny core, squeezing it.

Scientists have studied how these transformations affect observations. A puffed-up halo makes the star more pliable: it compresses and stretches more in a neighbor's gravitational field. This is measured via gravitational waves — ripples in space that were first detected by instruments created by Rainer Weiss. However, a dense dark core shrinks the star, which can be observed in X-rays using spectroscopy (analysis of radiation). It turns out that for light dark matter, its fraction cannot exceed about 11%, otherwise the star would become too large. Heavier particles are allowed in greater amounts.

This approach doesn't require precise knowledge of dark matter's nature: it works for almost any theory beyond the Standard Model of physics. Therefore, neutron star observations become a universal detector of invisible matter.