WWW.ASTRONOMY.COM 25and evolved over most of its history, they
know relatively little about the first sec-
onds that followed the Big Bang — and
next to nothing about the first trillionth
of a second. When it comes to how our
universe may have evolved, or to the
events that may have taken place during
these earliest moments, we have essen-
tially no direct observations on which to
rely. This era is hidden from view, buried
beneath impenetrable layers of energy,
distance, and time.
Our understanding of this period of
cosmic history is, in many respects, little
more than an informed guess based on
inference and extrapolation. Look far
enough back in time, and almost every-
thing we know about our universe could
have been different. Matter and energy
existed in different forms than they do
today, and they may have experienced
forces that have not yet been discovered.
Key events and transitions may have
taken place that science has yet toilluminate. Matter likely interacted in
ways that it no longer does, and space
and time themselves may have behaved
differently than they do in the world
we know.
With this in mind, many cosmologists
have begun to consider the possibility
that our failure to detect the particles
that make up dark matter might be tell-
ing us not only about the nature of dark
matter itself, but also about the era in
which it was created. By studying darkAlthough dark matter in
galaxy clusters typically
traces the ordinary
matter that radiates light,
ZwCl 0024+1652 goes its
own way. This massive
cluster sports a dark
matter ring (in blue)
spanning 2.6 million light-
years that appears largely
divorced from the visible
galaxies and gas. NASA/ESA/
M.J. JEE (JOHNS HOPKINS UNIVERSITY)