A dying massive star usually shrinks into a black hole — a trap from which even light cannot escape. But another finale is possible: a gravastar — a bubble of dark energy, the mysterious force pushing the cosmos apart.

Simulations of a collapsing cloud revealed: deep in the center, a tiny nugget of dark energy is born. It inflates, like a soap bubble in a vacuum. Upon reaching the critical radius where a black hole would normally appear (first calculated by Karl Schwarzschild), the expansion halts. Infalling matter hangs suspended on the bubble — a stable gravastar is born. The surprise: inside, space stretches as rapidly as in the first moments after the Big Bang — a whole mini-universe beneath the shell.

The trick doesn't always work. The star must be sufficiently 'fluffy': its mass-to-radius ratio (in special units) must be less than 3/8. Otherwise, collapse into a black hole is inevitable.