Massive surveys searched for objects with abnormally strong near-UV flux. These turned out to be either quasars or white dwarfs. To date, nearly 30,000 quasars have been discovered this way, and literally thousands of papers have been published describing their properties and distribution. Unfortunately, it seems as if all this vast work, spread over decades, may have been missing the point. It has recently become clear that many, perhaps most quasars have very red colours.
Nobody knows why.
These red quasars do not have strong UV fluxes, and so have been missed by all existing surveys. At present, very little is known about these red quasars. Their redness may be due to dust, though that isn't sure. They seems to have anomalous spectra and remarkably strong polarisation. None of this is well understood.