Clusters of galaxies are powerful gravitational lenses that can
greatly magnify other galaxies lying behind them. The phenomenon of
gravitational lensing causes magnification of the light of background
sources when light is deflected by the gravitational field, and galaxies
are distorted into elongated shapes called arcs when they lie near
curves called caustics. If a luminous star in the lensed galaxy lies
really close to the caustic, it will be magnified even more than the
average of the galaxy and may become visible to our best telescopes even
from a cosmological distance.
This magnification becomes interesting because it is affected by any small scale irregularities in the mass distribution of the lensing cluster. While some irregularities will be due to intracluster stars in the lens, the dark matter may also incorporate small-scale density fluctuations that can affect what we observe, through microlensing of these stars in background galaxies.