When I opened it with ImageMagick it appears to be a transparent PNG. Very clever trick, as it seems to use Twitters black background when viewing an image to achieve the effect.
So it's unrelated to the gamma trick in this article?
Yes, it is.
In that case, how does the room disappear?
The room is drawn with translucent black pixels that show up on any light background. The stars are drawn with translucent white pixels that show up on any dark background.