we could be the oldest civilization in the Galaxy

The reasons and scenarios listed so far have been rather tame. Here's
some more exotic explanations of the Fermi Paradox.

1. They are not interested in us: planetary and stellar-system sized
minds.

http://en.wikipedia.org/Jupiter_Brains
http://en.wikipedia.org/Matriosha_Brains

human mind: estimated 10^16 ops/second
human species: 10^26 ops/second
matrioshka brain: 10^42 ops/second

Such an entity would have no more interest in talking to us than we
would in "talking" to an amoeba. It could run simulations of us and/or
observe us from afar with solar-system sized telescopes, to know
everything about us it would need to know.

Interstellar expansion for these entities is not very profitable,
because the huge distances between stellar systems makes data
transmission very slow.

2. They don't expand into the galaxy, because they won't need to:
expansion into "inner space" (cyberspace).

Computers have been getting more and more powerful, smaller and
faster for the last 60 years (Moore's Law). Eventually they could run
simulations of all physical phenomena in the observable universe--
including us (technological civilizations). They could do this
instead of going to the fuss of going to see us. Related to (1).

3. They don't expand into the galaxy, because there are more
interesting places to go: other universes.

According to certain theories in physics, there are other universes.
Using wormholes a sufficiently advanced technology could tunnel into
these, or even create them with properties more sanguine for life
than this one.

4. They don't expand into the galaxy, because they have evolved value
systems beyond ours that are non-expansive.

We've noticed that more evolved creatures here on Earth tend to
make less offspring. Also, birth rates have been declining for the
advanced nations of the world.

A more advanced species might see expansion into the galaxy as just
a huge magnification of the problems they already have in their
planetary system--the endless cycle of population increase, and the
subsequent scramble for resources this makes.