  The concepts of negative gravitational mass and gravitational repulsion are
alien to general relativity. Still, we show here that small negative
fluctuations - small dimples in the primordial density field - that act as if
they have an effective negative gravitational mass, play a dominant role in
shaping our Universe. These initially tiny perturbations repel matter
surrounding them, expand and grow to become voids in the galaxy distribution.
These voids - regions with a diameter of $40 \h$ Mpc which are almost devoid of
galaxies - are the largest object in the Universe.
