What is darkness actually?? I heard people talking about anti photons..What are they?

Darkness is lack of light - the absence of photons; there is no need to talk about anti-photons. In fact photons are their own antiparticle ie photons and anti-photons are indistinguishable - so we don't really think of them in these terms at all.

I agree with imatfaal.

Darkness is lack of light. In order for an observer to observe and 'see' the darkness of a subject, that subject should not emit any visible light towards the observer. See for example in a recent Astronomy Picture of the Day the dark Pipe Nebula http://apod.nasa.gov/apod/ap121123.html