At the center of the Milky Way hides a supermassive black hole Sagittarius A* — 4 million times more massive than the Sun. Around it, as if on invisible ropes, stars whirl. According to Einstein's theory, orbits should slowly precess. But if the particle of gravity — the graviton — has mass, the rope stretches slightly, and the orbital precession grows stronger. Astronomers tracked stars S2 and S38 and compared their paths with computer models. They found no deviation from the Einsteinian scenario. So the graviton's mass — if it has any — does not exceed one-million-million-millionth of an electron's mass. This is so minuscule that such a particle's wave would spread across hundreds of solar systems — an invisible ocean whose ripples we notice only by the dance of distant stars. The same conclusion follows from gravitational waves from black hole mergers registered by detectors. Thus our Galaxy becomes a laboratory for testing fundamental laws.