Galaxy clusters work like a steamy lid on a boiling pot of soup. Their colossal mass bends space, like condensation droplets on glass, focusing light from distant stars into brief flares. These 'droplets' are tiny clumps of dark matter, possibly made of lightweight axions or microscopic black holes. When a star's image drifts across the sky and passes through such a clump, its brightness spikes dramatically for hours or days. This gravitational amplification is like when a wet lens suddenly sharpens details, but here, an entire star is magnified.

Telescopes like James Webb and Hubble catch these events from stars billions of light-years away (distance determined by redshift). Precise brightness measurements (photometry) and computer simulations allow scientists to reconstruct a map of dark droplets from the shape of the flare. Amazing fact: at such a distance, a star normally looks like a point the size of a coin on the Moon, but the magnification is so strong that astronomers can detect limb darkening, just like our Sun. This helps distinguish the nature of the clumps: fluffy axion clouds or dense black hole points. Early results show that dark matter is far grainier than we thought, and the mystery of its nature is closer to being solved.