The Milky Way
The Milky Way is the galaxy that contains the Solar System: a large barred spiral whose disk spans about 100,000 light-years, with roughly 100–400 billion stars and a total mass of about 1.5 trillion solar masses — most of it dark matter. Astronomers map it from the inside with ESA's Gaia mission, and test the laws of gravity on stars whipping around Sagittarius A*, the four-million-solar-mass black hole at the centre.