system
Given the following post, respond as if you're commenting on Reddit:
human
Is there dark matter in our galaxy or even solar system?

If dark matter is everywhere it stands to reason that it is also in our galaxy, possibly in our solar system and even near or on planet Earth.
gpt
Theory and observation suggests that dark matter seems to form in roughly spherical galaxy-sized blobs. There may be some amount of smaller scale structure, but the most established part is that there's something going on at a galaxy scale - even if you go with modified gravity instead of dark matter, these modifications need to take effect on a galaxy scale rather than a solar system scale.

The idea is that dark matter particles don't bump into each other, and therefore can't lose energy through collisions, so there's a limit to how far it can collapse. If you start with a uniform soup of dark matter, gravity will cause the dark matter to start to pull together into clumps. As the dark matter is accelerated into clumps, it speeds up. You reach a point where the dark matter's speed balances out the gravity of the dark matter clump, and the dark matter particles are basically all buzzing around in orbit within their common gravity. This balance happens at around the size of a galaxy, and is basically why galaxies are the size they are.

From observations, it's only on a galaxy scale that we see gravity deviating from what you can explain using general relativity plus the assumption that only visible things like stars produce gravity. So that also confirms the biggest effect seems to be going on at these big scales.

So yes, there is dark matter spread all throughout the galaxy.

However, even though dark matter has more mass than the stars and gas and stuff within our galaxy, its *density* is low. Stars and planets form from gas, and gas particles *can* bump into each other and lose energy. This causes gas to settle down into a disc, which is the least amount of motion you can have without breaking angular momentum. The stars then form within this gas disc, to form a disc of stars - although afterwards stars can get stirred up to form a fatter disc, or even a spherical "halo".

So what we have is a huge puffy ball of dark matter, and a thin dense disc of stars. When you're within the stellar disc - as we are - the mass nearby is totally dominated by stars and stuff. Dark matter has a lower density, it's just more spread out, which means that it adds up to more mass overall.

So yes, while there is dark matter within the solar system - it's spread out everywhere - the mass of dark matter within the solar system is less than the mass of the Sun.

Additionally, because dark matter is spread out smoothly over such a large scale, even if the density of dark matter was really high within the solar system, it wouldn't affect the Earth's orbit or anything very much. Instead, dark matter pulls the whole Solar System by the same amount - the Earth and the Sun are pulled the same amount - which means that, from within the Solar System, you don't notice any difference to your orbits.