They are more than a billion light-years away, and are more than one hundred thousand times fainter than the human eye can see. It is well known that massive black holes devour stars and gas. Black holes like this are called quasars, and were first discovered in the 1960s. But until now only blue quasars had been seen. An early theory that a shroud of dust made blue quasars look pink has now been discounted.
Dust clouds absorb blue light, leaving a yellow-red glow.
However, the precise shade to be expected can be accurately calculated and does not match the pink the astronomers saw. The BBC is not responsible for the content of external internet sites.