  Recent observations have shown that, only a billion years after the Big Bang,
the Universe was already lit up by bright quasars fuelled by the infall of gas
onto supermassive black holes. The masses of these early black holes are
inferred from their luminosities to be > 10^9 solar masses, which is a
difficult theoretical challenge to explain. Like nearby quasars, the early
objects could have formed in the central cores of massive host galaxies. The
formation of these hosts could be explained if, like local large galaxies, they
were assembled gravitationally inside massive (> 10^12 solar mass) halos of
dark matter. There has hitherto been no observational evidence for the presence
of these massive hosts or their surrounding halos. Here we show that the cosmic
gas surrounding each halo must respond to its strong gravitational pull, where
absorption by the infalling hydrogen produces a distinct spectral signature.
That signature can be seen in recent data.
