400+ billion stars per galaxy might make each one seem 'dense' but the distance between stars is enormous.
I have heard that it might be possible for one galaxy to pass through another without any stars colliding with each other. I don't have any idea if that is actually true.
The reason for this is because the dynamical systems of gravity, conservation of momentum etc. tend to pull approaching stars into orbits around each other or make them slingshot away after a close approach, rather than collide directly. Even though intuitively you feel gravitational attraction would make it so that they pull together, the mechanics tend to prevent that from happening. It's the same reason that, unintuitively, it takes a lot of energy to bring a rocket from a stable orbit down to Earth.
That's not to say that direct collisions won't happen, the circumstances will surely be there for them to happen, with all those millions of stars, just less likely than you'd think.
When two stars collide it's usually because two stars are in close orbit and something causes an orbital decay, such as one leaching matter from another, or another star passing close enough to disrupt it. This last point is probably more of a catastrophic risk here; even more so the possibility of a passing star slingshotting planets away into open space.
Source: this was part of my undergrad thesis.
In reality there will be always some sideways motion for each star relative to each other, especially in this kind of situation with a lot going on. The gravity of the bodies pull on each other as they approach, accelerating them towards each other, but their momentum is strong, and so gravity pulls them into an elliptical orbit (or hyperbola) and not into each other. There only needs to be a tiny relative sideways motion for this to happen.
In other words, gravity will not cause two stars passing very close to each other to collide, as one might expect. The stars really would to be aimed perfectly at each other to collide, which would be extremely unlikely with both stars feeling the faint pull of many distant neighbours.
I think I was being overly cautious when I suggested a direct collision was likely to happen. It's more accurate to say that an accidental confluence of the many complex gravitational forces and sheer number of stars could maybe allow collisions to happen somewhere in that system.
Dark matter is just a placeholder until we find whatever that "something" is, or a better model of why this is happening arises. All of that is predicated on the estimate of kinetic energy you inquired about
All of our observation tells us that whatever dark matter is, it doesn't interact with the electromagnetic field. That is, it does not interact with light of any frequency, so is completely undetectable to us. As far as we can tell, dark matter appears to not interact with anything other than gravity, which is pretty weird. We think that it doesn't even interact with itself; two dark matter particles should just pass through each other.
We know a lot about what dark matter isn't, but we're still pretty clueless on what it is
[0] https://en.wikipedia.org/wiki/Massive_compact_halo_object
[1] https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
If this missing mass were in the form of dust the entire night sky (relatively speaking) should be full of dust reflecting light and radiating infrared due to being lit by starlight for billions of years. But we don't.
If the missing mass were in rogue planets, brown dwarfs, or even cold dwarf stars there would need to be so very many of them that we should be detecting them by the millions with our current telescopes as they pass between us and distant stars. But we don't.
The better our telescopes get the more and more certain we are that the missing mass is not normal matter as we know it. We are getting really good at spotting dim objects (or their side-effects) even when hidden by the glare of stars. Normal matter but not radiating much energy just can't hide from us well enough to account for the missing mass.
> Although the galaxies will plow into each other, stars inside each galaxy are so far apart that they will not collide with other stars during the encounter. However, the stars will be thrown into different orbits around the new galactic center. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.
Yes there are a lot of stars. It's a big number. But there's a lot of space. That's a number that's in a whole different league.
For example, the whole solar system is about 2 light-day diameter. But it's 4 light years from the nearest star in any direction. Empty space is thus many orders of magnitude more than solar systems, never mind suns.
Sure there's a probability of a collision. But even multiplied by the number if stars, it's still really tiny.
Gravity tends to bring heavy objects together
Andromeda and the Milky Way are not colliding "like two pancakes on top of each other" but at an angle to each other
Also star surroundings are usually places where you can collide with a lot of stuff (asteroid belts, dust, etc)
We're not shooting a goal on a 4 light-year wide goalpost (btw that's the density on our vicinity, but on other areas the density is higher) but passing multiple stars on an environment that can be perturbed and heavy things attract each other
If they merge in 7 billion years, you think over the eg next 7 billion there won't be kinetic collisions? I'm skeptical.
Also watching stars getting disrupted by black holes would be similar.
Expected Number of collisions = Number of stars in galaxy B * cross sectional area of star in A / average area of galactic circle per star in A.
https://www.wolframalpha.com/input?i=%28number+of+stars+in+m...
says it comes to 0.5 - 1.0 (the uncertainty comes from number of stars in the milky way)
My assumptions are bad enough that it could be off by a factor of 100 one way or the other (there should be a few factors of 4/pi, it looks like Andromeda is about twice the size of the Milky Way, the average star is smaller than the sun, no stars are point like, gravity probably does something, stars are much more densely packed towards the galactic center, I'm calculating the result one galaxy passing through another, not a merger in which they might partially intersect more than once).
But the orbit of Jupiter is still a lot bigger than the size of the Sun. To give numbers: The radius of the Sun is 700km, the radius of the orbit of Jupiter is 7*10^8 km (approx, it varies a bit), and the distance to the nearest star is 4*10^13 km.
700 Mm, or 700000 km.
If you reframe the question as "will the collision of the galaxies cause problems for some hypothetical civilizations who may be living there" the probability of that is simple, it's 1. The good news for such civilizations is that they'll have literally hundreds of thousands of years of warning to deal with them. Planets may have problems but if a civilization is based on space stations and other off-planet structures they'll hardly notice the problems since they'll be so slow to occur.
> […] if the Sun were a ping-pong ball, […] the average distance between stars […] is analogous to one ping-pong ball every 3.2 km (2 mi).
Intuitively this visualization actually makes it seem like stars are pretty close? Usually with galactic dimensions it’s hard for our mere monkey minds to grasp the scales but this is actually pretty easy to imagine.
A table tennis ball is 40mm in diameter.
That makes the sun about 34,750,000,000 times bigger than a pingpong ball.
Edit to add: length, not volume