Planets, stars, galaxies, clusters, everything in the universe is spinning/rotating. Why?
In: Physics
The answer is conservation of angular momentum. If you have a big stationary cloud of gas, for one bit to start spinning the other bit also has to start spinning (in the opposite way). This is essentially Newton’s 3rd Law at work (for angular systems).
However, that’s just a one layer deep explanation. It explains why lots of things are spinning now, but doesn’t really explain ***why*** things were spinning in the first place. The ELI5 answer is we don’t really know. The general consensus at the moment is that the universe is probably isotropic, (i.e.: zero net angular momentum, no preferred universal direction). The milky way has a distinct spin direction, but summing over the entire universe there probably isn’t. General Relativity predicts that non-isotropic universes would probably allow closed time-like curves (fancy physics speak for “time-travel would be possible”, see [Gödel metric](https://en.wikipedia.org/wiki/Gödel_metric) as an example of a net non-zero angular momentum universe if you’re familiar with GR already). We’re pretty sure that it doesn’t allow that, though. Or at least if it does, then we’ve got more pressing issues to deal with than whether the universe is net rotating. I can’t find it now, but I read a paper a few years ago that was posted on a similar question in /r/askscience that calculated an upper limit for the net rotation of the universe and it was something stupidly low.
The idea that the universe is isotropic (and homogenous) is called the [Cosmological Principle](https://en.wikipedia.org/wiki/Cosmological_principle) (that wikipedia article has a warning about being too technical at time of writing so assuming you’re on ELI5 because you aren’t a physics expert, it might not be massively useful to you). It’s not been proven and is actually contested fairly frequently though. Previously, the apparent isotropy of the intensity of the [Cosmic Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) was used as justification for the Cosmological Principle, but 2013 data from the ESA’s *Planck* mission has indicated anisotropies in the CMB, as well as a bunch of other analyses from other missions, so this might be out the window as well.
Ultimately, even if the universe is net-rotating, it won’t be significant to the point of causing the pockets of rotation we see, so they probably come from chaos (in the maths sense). Small chaotic fluctuations in local angular momentum get amplified into larger scale but still localised pockets of distinct and significant spin (e.g.: galaxies). Which is just fancy science speak for “idk, man it just does, okay?”.
TL;DR: Because Isaac Newton said it was, then Einstein said it might not be, then Friedmann, Lemaître, Robertson, and Walker all said it’s not, then Gödel said no the first guy was right, and now everyone’s just confused.
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