STAR TRACK The Gaia spacecraft (illustrated above) is collecting data on position and motion for about 1 billion stars in the galaxy. The data may help resolve controversy over the pace of the universe’s expansion.
Milky Way: Serge Brunier/ESO; Gaia: D. Ducros /ESA
For as long as humans have wondered about it, the universe has concealed its vital statistics — its age, its weight, its size, its composition.