glass turns out black?

I followed the tutorial at
but my glass kept coming out extreemly dark. I’ve seen other pictures on the forums and this seems to be pretty common.

I want to make a glass cube that looks smooth, slightly reflective, and is not black… kind of like a glass cube in a well lit-room.

I’ve seen tons of pictures that were very bright, but most of mine come out dark.
(click for .blend file)

Any tips?

in the yafray tab there is a “depth” bar. increase depth value to more than one - i bet this is the problem.