Bao Wang

A distant radio signal travels toward us through gas, and the more gas it encounters, the more the signal is 'slowed down.' Astronomers use this to weigh the invisible envelope of our Galaxy: its halo. But if the halo is uneven, like a thick scarf draped over the shoulders, measurements in different directions will differ.

A massive star ends its life in a spectacular collapse: the core compresses into a neutron star or a black hole, while the outer layers fly off in a dazzling supernova.

Half a century ago, Fritz Zwicky noticed that the visible matter in galaxy clusters was catastrophically insufficient to explain gravity.

The universe is like a vast invisible scaffold. We don't see it directly, but we know it's there: galaxies and clusters hold together thanks to it. This scaffold is dark matter. The most popular version says it is "cold," made of heavy, slow-moving particles.