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[–] SwiftLion 0 points 10 points (+10|-0) ago 

"Explained" is a strong word. The other three forces have been UNIFIED.

Ultimately, we have this guess about the way the universe should be. It's maybe better described as a prejudice: humans appreciate elegance and simplicity. We believe that theories with fewer "moving parts", or simpler components, are "more true" or "better". Of course, this isn't the only test that exists--I can tell you, "the whole universe is made up of little tiny T-shaped bar magnets", and that theory may be very simple, but if it doesn't make correct predictions about how stars work, it'll be thrown out.

Matter as an Example of Simplification as you Drill Down

To be fair, it's true that things seem to become simpler as we "drill down". If you start off by trying to categorize "substances", you have billions and billions and billions of options. We even have neural networks that try to figure out the properties of various new substances to identify ones we like. But, what's more elegant than that? Elements. With only about a hundred or so entries, you can categorize all of those billions of substances as "arising from" elements. We actually thought, for a long time, that a correct "theory of the universe" involved around a hundred different kinds of "atoms". We called them "atoms" because we thought they couldn't be subdivided. Then, after that, we found out how to smash those apart, and discovered that ultimately, they're made of only three kinds of things: electrons, protons and neutrons.

Strangely, when you smash THOSE apart, you get quarks, which are weird--we step up from only three types of particles for the "elementary particles", to a much larger spectrum for quarks. You have the six flavors ( up, down, top, bottom, charm, strange ) and then three 'colors', but whatever.

Fundamental Forces

We should take just a second and clarify what we mean when we say "explain", too. Nobody can actually tell us WHY the electroweak force exists. We can only talk about 'what' it is. Right now, we have the 'Grand Unified Theory', which describes charge. The only thing is, it actually doesn't predict the existence of gravity.

Because we value elegance and simplicity, the idea that there are two, completely unrelated forces seems irksome to us. And that's the reason we say gravity is "unexplained": we EXPECT, we HOPE, that there is a way to describe charge so that gravity arises naturally as an aspect of it. This would be called a 'Theory of Everything', because it would explain a fundamental feature of the universe: "Here is what 'force' is". Then, all the forces we observe in our daily lives could be described as "aspects" of the fundamental force.

It may turn out that we have "explained" gravity just as well as it's gonna be explained. Maybe it turns out that there's not gonna be a unification at all, and the universe DOES just have two fundamental forces: charge, and gravity! But we just have a suspicion that it isn't quite so, and that there IS an explanation for gravity as an aspect of charge. And so, we call gravity "unexplained".