What existed before the Big Bang?  What are the cosmological speculations?

To a cosmologist, this quesiton is a non-starter.  The big bang is
defined as the beginning on time.  Therefore, there is no such thing
as "before".  It's not a very satisfying answer, but it makes sense.

Here are a few thoughts:

1) All the matter of the universe was condensed in a small ball since
the beginning of time until it exploded in the big bang.

2) The big bang has happened many (possibly infinitely) many times. 
The universe expands after the big bang and eventually gravity pulls
everything back in to the center... some time after everything has
returned, the bang happens again.

3) God created the big bang, so the universe did not exist before that.

4) There was no physical matter at some point in time, only energy
fluxuations.  At some point this energy begand to transition to matter
that was attracted to the other matter that was created (due to
gravity) until the matter became too massive to contain itself and
BANG.

5) The big bang is so 1990, look up some new theories.

**Sorry, the fifth one just slipped out.

Although this was posted in response to another question I believe it
answers your question.

----------
Takamasa Takahashi of St. Norbert College adds a few comments:

"Many scientists do not ask 'What came before the big bang?' because
it is beyond the scope of our physical theories. Usually we consider
the big bang to be the beginning of time and space, and so it is
meaningless to ask what existed before or what lies beyond the
expanding universe. Because space itself is intimately connected with
matter in the universe, as matter was created in the big bang, so was
space. There is no 'empty space' that the universe is expanding into.
Answer posted on October 21, 1999

http://www.sciam.com/askexpert_question.cfm?articleID=000CBA20-7D6F-1C72-9EB7809EC588F2D7&catID=3&topicID=2

The so-called "big-bang theory" was postulated by some lazy scientists
without having to explain "how the orderly arrangements of things"
came about.  It lacked facts, defied logic and intelligent arguments. 
I am surprised it still makes today's headlines.  Kind-a-dumb!

triumfdoogooder,

   On the contrary, the Big Bang theory is, in fact, the synthesis of
facts (observed properties of the world) with very demonstrably
reliable physics. There's nothing lazy about it. Indeed, it's lazy to
simply dismiss it and lean upon appeals to theology, as one with a
weak leg leans upon a crutch.

   There's also *nothing* in Big Bang cosmology which categorically
precludes a theistic agent being present in the Universe. The Big Bang
theory is *only* a statement as to an orderliness present in the
observations. Given that the Universe is undergoing an approximately
uniform expansion, an extrapolation backward in time necessarily
specifies the Universe being in a much less rarified, higher
temperature state. In addition, there's *nothing* in Big Bang
cosmology per se which makes any claims as to the origin of the
expansion, nor is there any pretense to knowledge that there either
was/was not time previous to the expansion. The theory is ONLY about
the dynamics of the Universe since a certain point in time, at which
the problem becomes tractible.

APPLAUSE for qed100!

You will have to think with fresh mind. Before the Big Bang there was
nothing if anything was there, we can not tell about it because that
would be pointless and dimentionless thing, but the explosion of that
created many dimentions like length, width, mass, time etc. However
this approach is not very convinient but we
dont know anything else. That celestial event, the big bang produced
the matter and ever expanding galaxies.

A Pumpkin

The events "before the Big Bang" are in detail described in this
"timeline of the Big Bang" (i.e. "according to the scientific theory
of the Big Bang"):

http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang

On the 7,000,000,000,000th day Man created god(s)