  The formation of the first massive objects in the infant Universe remains
impossible to observe directly and yet it sets the stage for the subsequent
evolution of galaxies. While some black holes with masses > billion solar
masses? have been detected in luminous quasars less than one billion years
after the Big Bang, these individual extreme objects have limited utility in
constraining the channels of formation of the earliest black holes. The initial
conditions of black hole seed properties are quickly erased during the growth
process. From deep, optimally stacked, archival X-ray observations, we measure
the amount of black hole growth in z=6-8 galaxies (0.7-1 billion years after
the Big Bang). Our results imply that black holes grow in tandem with their
hosts throughout cosmic history, starting from the earliest times. We find that
most copiously accreting black holes at these epochs are buried in significant
amounts of gas and dust that absorb most radiation except for the highest
energy X-rays. This suggests that black holes grow significantly more than
previously thought during these early bursts, and due to obscuration they do
not contribute to the re-ionization of the Universe with their ultraviolet
emission.
