Proving a Negative
Technically, it is impossible to prove anything; there is always the possibility that everything we know is wrong, or a lie, that we are part of a collective subconscious creating a reality alternate to that of the true universe, without the same rules and laws of physics; as long as there is still a shred of doubt that we don't possess all the knowledge in the universe, there is always the potential we are wrong. Even if we did hypothetically obtain all the knowledge in the universe, there would always be the question of how we knew, for certain, that we knew all the knowledge in the universe (all the knowledge in the universe could be knowing that we know, although this would still be impossible to prove). In essence, it is impossible to, with absolutely certainty, prove anything.
We can get within reasonable doubts, however. Negatives tend to be very difficult to prove; that is, the absence of something. While one could not for instance, prove the existence of the universe, one could not disprove it either. While it would be easy to provide evidence for it's existence, evidence for it's non-existence could perhaps be nothing, which would essentially provide no proof. It is true that an absence of evidence is not evidence of absence, however, evidence of absence is possible to provide. It is not true that negatives cannot be proven, nor is it true by the same token that a lack of evidence is proof of existence.
How would one prove a negative? To prove God doesn't exist, one would seemingly need to first obtain all the knowledge in the universe, rooting out the existence of God by proving that, in the body of evidence of everything, God was not there. As described above, since this is essentially impossible, it would be impossible to prove God didn't exist unless we had completely knowledge about the universe. However, proof God exists would not require proof of all of the universe; thus, the location, the actions, the current state of every atom, higgs boson, curve in spacetime is not necessary to prove the existence of God, while it is necessary to prove the lack of an existence of God. Some will fallaciously use this as an excuse to state why they cannot prove their point, or even worse suggest that the burden of proof lies on the one claiming the existence of God (although claiming the non-existence of God ironically also demands proof, no less so than claiming cats do not exist). The burden of proof rests on all of mankind; believing in no God without proof of a lack of existence, is just as illogical as believing in a God without the proof of existence. Without evidence, we can make no assumptions. It is incorrect, and illogical then to assume anything without proof, and given that the truth remains the truth regardless of human knowledge, the burden of proof is present on everyone, as no-one is abstained from the truth.
Still, what of Negatives? How would someone prove the lack of existence?
While this is more or less impossible, it is possible to prove within a reasonable shadow of a doubt that something does not exist. How would one for instance, prove something was not the color green? By virtue of it's exclusionary properties, we can conclude that something could not be two colors at the same time, I.E. red and green. Since something can only possess a single color, at least according to the electromagnetic spectrum, only possessing on wavelength and frequency, there is no possible way that something could in fact be green and red at the same time. If a factor excludes information, in that by virtue of being something, it cannot be something else, we can provide evidence for the existence of something, which by that nature invalidates the existence of something else. In physics for instance, two objects cannot take up the same space at the same time; as a result, by the presence of matter, we can prove that no other object could possibly there, thus proving a negative. We can disprove a wide range of qualities by innate truths pertaining to a subject. While this is still impossible to prove (since we cannot prove anything beyond 100% certainty, given that there will always be doubt), this evidence, is considered acceptable, in theory could disprove the existence of a particular attribute. This can be applied to many things; based on biology, a creature could not be both a cat and a dog at the same time. By proving it is a cat, we disprove that it is a dog; by proving for instance, Big foot was actually a man in a suit, or a bear, we prove that, in this particular case, Big foot does not exist. Thus while we can never truly prove any point, it is hypothetically possible without proving everything in the universe first that something does not exist.
Therefore it is within the realms of possibility to realistically prove or disprove a negative. It is hardly an excuse for the lack of evidence for your position, whatever it may be.
