The Hubble Telescope captured images of these galaxies surrounded by glowing green clouds. They’re remnants of major astronomical events, but the amazing thing is what’s lighting them up.
The clouds hovering around the far edges of those galaxies are made of oxygen gas. They’re the deceptively peaceful-looking wisps left over from massive galactic collisions - collisions that left long trails of gas streaming far away from the centers of their galaxies. The clouds are tens of thousands of light-years long, and linger far away from the galactic centers.
The fact that they’re far away is probably the reason they’re still glowing. When oxygen is exposed to large amounts of energy, it can have its electrons ripped away, making it into an ion. Ionized oxygen glows green. But why can’t we see the energy that’s making it glow?
The source of that energy has gone dark. It’s a quasar, a massive black hole that ejects energy as it takes in mass. The quasars in these cases have stopped emitting energy, but the energy that they have emitted just keeps traveling. After tens of thousands of years, the energy hits these oxygen clouds, ionizing them and lighting them up.
So while a quasar itself has gone dark, its energy keeps going through the universe, lighting up one object after another like a string of blinking lights.