As you're probably aware, we know of three elemental damage types in Destiny: Solar, Arc and Void. Weapons can deal these damage types, and each Subclass is assigned one - for example, the Gunslinger uses Solar damage. Now, Arc damage is simply electrical energy, while Solar damage is incendiary, 'harnessing the power of the sun' for heat- and fire-based attacks. However, info on what exactly Void damage is and how it works is non-existent. One could assume that Void energy is some form of power generated by the Traveller, but I have a different theory. Typically, a void can be a physical location, perhaps outside of normal space and time. The first Warlock Subclass is called 'Voidwalker', who are allegedly 'not bound by the laws of space and time'. If a Warlock can walk there, is the Void a real place? Might we be able to explore it in Destiny? Who knows... I'd love to hear your ideas :)
I think "Void" is having to do with the Darkness. Void in literal terms is nothingness. Which is comparable to darkness. I think it may be a special, dark ability.