  Models for the formation of galaxies and clusters of galaxies require strong
feedback in order to explain the observed properties of these systems. We
investigate whether such feedback has observational consequences for the
intergalactic medium, as probed in absorption towards background quasars. A
typical quasar sight-line intersects one proto-cluster per unit redshift, and
significant feedback from forming galaxies or AGN, heating the proto-cluster
gas, will result in a large clearing of reduced absorption in the Lyman
alpha-forest. Such a gap could be detected at redshift > 3 when the mean
opacity is high. Feedback from Lyman-break galaxies in proto-clusters can be
probed by the absorption lines produced in their winds. Strong feedback from
galaxies has a major impact on the number and properties of absorption lines
with neutral hydrogen column densities of 10^16/ cm^2. This feedback can be
probed with multiple sight-lines and by studying the unsaturated higher-order
lines of the Lyman series. Galactic winds from dwarf galaxies should break-up
into clouds, in order not to over produce the number of absorption lines. These
clouds can then coast to large distances.
