"Why is there something, rather than nothing?"
Try imagining "nothing". Space, time, and the Universe are "something" so subtract those as well. Forgetting that you know something exists (i.e. using a priori reasoning), its seems more logical that nothing should exist, rather than something. Why? Something requires explanation. Like a chicken/egg dilemma, "What created the something, that created the something, that created... etc". It doesn't matter whether you explain a first cause with a physical nature (i.e. Big Bang) or a sentient one (i.e. God); any such explanation demands further explanation. So, "Why isn't there nothing?" is a riddle for theologians, philosophers, scientists, and anyone curious about existence.
Our Universe is "something" but perhaps the issue isn't so black and white, and the problem requires thinking differently. Our brains evolved (for spear chucking and what-not) to think in three-dimensional space per Newtonian physics and to see a sliver of EM spectrum. However, just think about relativistic frames near black holes or quantum probability waves to realize how challenging reality can be to comprehend. Space, time, and matter defy common understanding at these extremes. Yet cosmologists keep peering out to estimate the size and age of the Universe, and physicists keep colliding atoms to find the smallest string or boson. While important, these scientific efforts can never find the smallest, largest, oldest, or farthest limit (because what's beyond that?). As the history of science (and surely modern science as well) is rife with anthropocentric blunders, perhaps knowing the answer can only be reached by forgetting what we know.
For such thought experiments, Occam's Razor (the principle of parsimony) is an intuitive rule of thumb suggesting we find truth in what requires the fewest assumptions. As described earlier, "something" is difficult to explain because it raises more questions than "nothing". For a similar reason, "a universe with 8.3 zillion units of energy" is more difficult to explain than "an infinite universe". As incomprehensible as infinity is, it requires less explanation than an abrupt end (raising questions: why that size? what's beyond it?). Similarly, it's simpler to assume the Universe has a net zero energy, than that it has some arbitrary amount of energy violating the first law of thermodynamics. Nothing(zero) and everything(infinity) are parsimonious concepts, so perhaps these are part of the answer.
Here's one idea that might stretch that spear-chucking brain a bit. What if our flat infinite net zero-energy universe is actually just one version of "nothing"? That's not to say we don't exist, but simply that 'everything that can exist must exist, so long as it balances out'. For example, take the equation 0=0. Potential balanced terms exist to produce infinite combinations of this same equation. One combination (such as our universe) balances positive energy(mass) with negative energy(gravity). Infinite variations balancing "energy" to nothing have potential to exist. Each such "universe" would be separate and independent with its own emergent time and space (if it had such properties). This is different from the concept of a Multiverse in which universes are "parallel" (permitting Star Trek transporter mishaps with evil goatee'd Spock). That's not to say there is a cartoon universe with pink unicorns, but merely that if it can exist it must exist. When thought of in this way, the original question turns on it's head. It becomes more parsimonious (although mind bending) to state: "If there is nothing, there must be all versions of nothing... meaning everything." Now, wouldn't that be something!