Re: Does true darkness exist or is it just the lowest intensity of light?

It's just a question of semantics, isn't it?
Well, if I define "true darkness" to be the total absence of
light, then maybe what you're asking is: Is there such a
thing as true darkness?

The answer is: no, not in this universe.  Our universe is flooded
with radiation (light) and matter (particles) of all descriptions
and energies, pretty much anywhere you go.  But even if you could
imagine constructing a chamber which was shielded from all
radiation and energetic particles, and somehow shielded out all
neutrinos, and was cooled down to "absolute zero" so that the
walls of the chamber didn't emit thermal radiation -- even then,
if you looked around, you would see a tiny bit of light, which
comes from the production of "virtual pairs" of particles.
This is due to the nonzero "vacuum energy" -- the cold vacuum
of space is constantly seething with virtual particles which are
created in pairs, and annihilate each other again.

The resulting bit of light is extremely far below the limit of
the human eye's sensitivity, or of the sensitivity of any
known instrument.  In fact, if you were trying to look with 
your human eye, you would guarantee that it wasn't dark
because your body is very hot, and emits plenty of its own radiation
(mostly infrared -- which can be picked up with high-tech
surveillance systems).  You can't have an observer who is non-emissive.

What if you had an observer inside a black hole inside the chamber, 
so that she could see out, but none of her own thermal radiation could
escape the black hole into the room.  Even then, the black hole would
emit radiation that it picked up from the vacuum (this is called
"Hawking radiation" and exists because even the black hole has a
temperature of sorts).

Anyway, who'd want to live in a totally dark universe?

-Aaron