What is Dark Matter? We know dark matter is out there—but what
is it? An invisible army of black holes? A cosmic graveyard of
burned-out stars? A swarm of rogue planets that roam the depths
of interstellar space? While examples of objects like these have
been observed, we now know that they can’t account for the
enormous mass of dark matter required to explain why galaxies
rotate so fast. Following Sherlock Holmes’ dictum that once you
have ruled out the impossible, whatever remains, however
improbable, is the answer, scientists have been forced to
conclude that dark matter is an entirely new form of matter,
never before observed. Here is what we think: Every galaxy is
engulfed by a cloud of dark matter particles that extends far
beyond that galaxy’s visible edge. Each dark matter particle is
electrically neutral and has a mass tens or thousands of times
that of the familiar proton. Finally, there is a lot of this
dark matter. Our best estimate is that there is about five times
as much dark matter as there is