The actual big short (haven't seen the movie) was on a credit default swap index[1] (it was on one of the CMBX series which have commercial mbs as underlying[2]. There are also RMBX series for residential mortgages). CDSs are a swap (ie they are synthetic by definition) where one person is long the risk (short the protection) and one person is long the protection (and therefore short the risk). So the concept of who is long and short can be a little confusing.
[1]Source: worked at Goldman as a strat (quant developer) on the mortgages desk at the time. I knew and worked with pretty much all of the people involved although I was based in London. The actual person who came up with the research behind the short idea was a fellow strat and brilliant econometrician[3] and most of the trading guys fought him like hell over it only to later found hedge funds and pretend that it was their idea all along.
(Edit: Context: the movie "The Big Short")