Recently, astronomers spotted three interstellar comets coming from deep space. One of them, 3I/ATLAS, turned out to be unexpectedly large: its nucleus is several kilometers across. This got scientists thinking: there might be countless such icy wanderers throughout the Galaxy, with a combined mass comparable to all visible stars combined, around 50 billion solar masses.

This much ordinary matter changes the picture of mass distribution. For a long time, the fast rotation of galaxies was explained by the presence of invisible dark matter, a hypothesis proposed by Fritz Zwicky and confirmed by observations by Vera Rubin. But now there's an alternative: we simply overlook a huge amount of ordinary objects, like weighing a bag without realizing it's stuffed with thick books. The weight seems to appear from nowhere, but it's actually hidden in plain sight.

New calculations suggest that the density of dark matter near the Sun could be almost half of previous estimates. To be fair, explaining all rotation with just comets would require an absurdly large number. But the discovery itself is a reminder: we still have a poor idea of how much ordinary matter lurks in the interstellar darkness. And these comets themselves are time travelers: unchanged for billions of years, they even witnessed the birth of our Sun.