[ALS]

"The currently accepted theory indicates that this phrase is meaningless. Time started with the BB."

prove it

[BMCDA]

Asking what was before the big bang makes as much sense as asking what is north of the north pole. The north pole is the point on a sphere with the lowest latitude so there are no points with a lower latitude.
And it's the same with time: the big bang didn't "happen" in space and time but it was the beginning of space and time (IOW the point with the lowest time coordinate) and therefore "before the big bang" is a meaningless phrase as Doctor Stochastic already said.

For further info you may want to read Ned Wright's Cosmology FAQ.

[Doctor Stochastic]

The best evidence (note that theories are never proved, only supported or opposed by evidence) is the microwave background.