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I have posted this in another forum and got some interesting answers.
Photons would interact with each other if we were to have such conditions.
An example of this sort of photon-photon interaction is the creation of e+ e- (electron/positron) pairs out of the vacuum. If you put enough photons in a box, you'll exceed the Schwinger critical field, and you'll start to see electron-positron pair production in the box. So the pressure will rise and the box will eventually explode.
So that was the answer of the original question by darke. I hope you understand it.
Basically we start with massless particles and end up generating mass in the enclosed vacuum till presure rises beyond critical point.
Here is a link to other views regarding the question.
http://www.physicsforums.com/showthread.php?t=103186