Like all ‘Why?’ questions, that title has a built in assumption. Unlike ‘What?’ which is a query about the identity of something and can be answered with ‘This’ or ‘That’, and unlike ‘Where?’ which can be responded to with a location, or ‘How?’ which asks for an explanation of a causal mechanism that can often be supplied, ‘Why?’ either leads to an infinite regression or often simply cannot be answered because the inbuilt assumption is incorrect.
What am I talking about? Well, ‘Why?’ asks for a reason, purpose or intention and there may not be one! Let me give you some examples: ‘Why did that coin land heads up?’ ‘Why did that gull poo on your head?’ ‘Why did Mexico City suffer an earthquake?’ The answers to all three are simply that we don’t know. Random events like those have no explanation; ‘Why?’ is just a silly question in those cases. Where ‘Why?’ can be answered because there is a purpose or reason, the answer is not necessarily conclusive; it merely prompts the next ‘Why?’ We have all heard children annoyingly exploiting this; ‘Why do I have to get up?’ ‘To go to school.’ ‘Why do I have to go to school?’ ‘To learn.’ ‘Why do I have to learn?’ ‘To help you cope with life.’ Etc. Eventually we have to resort to an emphatic, ‘Because I said so!’
Now let’s look at the particular question in the title: ‘Why is there something rather than nothing?’ Well, there is plenty of evidence for something: there’s a very great deal of observable matter, forces and radiations in the universe, but what exactly do we mean by nothing? We may be able to conceive of a total absence of everything, but does it actually exist?
For a start, ‘nothing’ has never been observed. Space is not ‘nothing’. It contains light photons, heat and many other wavelengths of the electro-magnetic spectrum, which amount to an average temperature of 3 degrees Kelvin not Absolute Zero. It also contains thinly dispersed hydrogen atoms and other particles, including virtual particles that pop into and out of existence all the time. It’s not even a perfect vacuum. We’ve tried to make ‘nothing’ here on Earth and, although we have achieved a much better vacuum than space (we can exclude light for one thing), we can’t keep out neutrinos. They pass through everything. So, as far as we know, ‘nothing’ does not exist – it’s just a concept.
The occasions when ‘Why?’ can be answered with finality are when it is applied to man-made objects or actions. That’s when it really means ‘What for?’ Examples: ‘Why is there a road outside?’ ‘For our vehicles to drive on.’ ‘Why do we wear clothes?’ ‘To keep us warm or protect us from sun damage.’ ‘Why did you stop suddenly?’ ‘To avoid running over that hedgehog.’ Humans initiate purpose, and so do other forms of life: ‘Why do daisies shut their flowers at night?’ ‘To protect them from cold.’ Organisms are purposeful, inorganic matter and processes are not. Seeking a purposeful explanation for inanimate natural events like the origin of the universe, or claiming that it must mean Godunnit is a Category Error.
In their efforts to confirm the existence of ‘god’, theists are driven to postulate that ‘nothing’ must have preceded the something of the universe, in which case its creator must have been outside of nature in a ‘Supernatural Realm'. Is ‘god’ nothing then? I could go along with that! But even that notion has a problem since it requires a time before creation for ‘god’ to have existed in (eternally?) and all the evidence suggests that time began along with the origin of space. There was no before.
Of course, we can still conceptualize ‘nothing’ and it is a useful construct. Maths would be very limited without zero but, until we find some real nothing, it will remain just an idea. A better question might be, “Why is there no evidence for nothing?” Answer: we just don’t know.