We are unable to propose the existence of nothing, because nothing is impossible for the human mind to comprehend in a similar way to the fact that infinity is impossible to comprehend fully. Human thought and analysis, as I understand it, is based off of the sensory inputs we receive.
Even when comprehending non-material ideas such as justice and mathematics there are strong roots to physical examples and phenomenon. In fact, even if you were to come up with a completely nonphysical example, I would argue that the sensory inputs we have received throughout our lives have influenced our thought patterns enough to play in to the analysis of that given question as well. I would furthermore go on to argue that we never really deal with ‘nothing’ even in the lesser extent of absence of a certain thing. As we define the absence of something through the definition of that something itself, and if there was not the something, we would have no measure of it, and therefore would be unable to deny it’s existence.
At the very least I can tell you that I have been unable to imagine nothing, even in the extremely limited capacity of the end of my own existence, and that my attempts to have been both frightening and unfruitful. They led to my attempted understanding of time and how anything can really exist in this strange flux of the universe.
However, despite my failure to answer your question, I can tell you that I am extremely appreciative of the fact that something does exist, and that for me, at least, that is enough.