Bow down before dark matter: that is one of the messages of 20th-century astronomy. Some unseeable material rules the cosmos, and ordinary matter is just along for the ride. It sounds like the culmination of the Copernican revolution, the ultimate displacement of humanity from a central role in the grand scheme of things. But lately researchers have been thinking that the lesson in humility has gone too far. What dark matter demands, ordinary matter doesn't always obey meekly.

Inklings of the spunkiness of ordinary matter have emerged over the past decade as observers have peered deeper into space and therefore farther back in time. According to the standard dark matter scenario, galaxies should have formed hierarchically: subgalactic scraps came first and slowly consolidated into full-fledged galaxies. Yet many galaxies seem to have jumped the gun: they were too big too early. "The mass assembly of massive galaxies is extremely and remarkably rapid--and much more rapid than is seen in the models," says Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

This article was originally published with the title Growing Pains.

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