They do! Or at least, we have very, very
good evidence that they do.
Part of the problem with detecting black
holes is that they, well, black: they emit no
light, so a black hole by itself is impossible
to see (unless you get close enough to it to
detect the very strong gravity from it).
However, if any ordinary matter gets close
enough to a black hole, it will spiral into it
and heat up due to friction with itself.
Because of the black hole's intense gravity, the
matter can hear up a lot, to the point of
emitting very strong jets of radiation. These
objects are called quasars, and we have observed
them at the centers of many galaxies. Although
the black hole causing them is impossible to see
directly, the evidence very strongly suggests
that they are caused by black holes.
In addition, we've also observed some of the
stars near the very center of our galaxy, and
discovered that they seem to be orbiting a very,
very massive invisible object - to the best of
our knowledge, this invisible object is a black
hole. In fact, evidence seems to suggest that
every single galaxy has a supermassive black
hole at its center.
Finally, there is hope that eventually we'll
be able to detect black holes directly in the
coming years through detection of their
gravitational waves. It turns out that
according to Einstein's theory of gravity,
accelerating massive objects should emit
gravitational waves - these are basically
ripples in spacetime. There are currently
experiments running (the biggest one being LIGO -
the Laser Interferometer Gravitational-wave
Observatory) which are designed to detect these
gravitational waves when they pass through the
Earth. One of the possible sources for these
gravitational waves is the collision of two
orbiting black holes, and LIGO scientists hope
to be able to detect these waves in the near
I hope that clears it up a bit!
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