Light is a visual effect commonly produced by heat sources or photofluroscent means. Darkness, as the other side of that coin, is the abscence of light, or, when you consider it, the 'normal' state in the abscence of any light source.
As far as I can tell, neither possess the ability to physically injure someone, though a heat source generating light may do so. Insofar as this, Ragione was partially correct in his usage.
The answer lies more in what the terms represent rather then the terms themselves, wheras Light represents a positive force, and Dark a negative. Using this analogy, if an equal amount of the two forces are used against one another, they cancel one another out. However, and I cannot stress this enough, neither are an 'element'. An alignment, perhaps, or an ability affiliation, but they are not elements. Such nonsensical ideas are propagated by the likes of pokemon, where an elemental archetype indicates the strengths or weaknesses of everything within that archetype.
Light and Darkness are not elements.
And they are by no means indestrutible. As states of the visual spectrum, they have no use in combat. As interprative 'titles', they can be cancelled by application of the other, or, as any 'elemental' attacks should be, ignored completly as an 'element' and dealt with according to the nature of the attack itself, and not that which it is comprised of.
No, really, I've got no time nor sympathy to waste on those who fall back on the crutches of 'elemental' characters in place of any actual creativity.