How do we know dark matter exists if we can't see it?

[Decentchat]

Our friends at W.A. Gayle Planetarium in Montgomery, Alabama, wonder, "How do we know dark matter exists if we can't see it?"

We're used to thinking about the universe in terms of what we can see. When we look at the night sky, we see stars, planets, and the Moon shining against the deep black background of space. When astronomers look at the sky with powerful telescopes, some that can detect light our eyes can't see, many more stars and galaxies show up.
Hubble telescope deep field.

All those bright objects the astronomers and we see are made of matter. We know that all matter attracts all other matter through a force called gravity. Earth's gravitational force is what keeps us on the ground, since we too are made of matter. Pole vaulters, elevators, planes, and rockets have to work hard to overcome the force of gravity. The same force that holds you and me to Earth's surface also keeps Earth-orbiting spacecraft and even the Moon from flying away. The Sun's gravity holds Earth and all the other planets, comets, and asteroids in our solar system in their orbits around it.

Scientists understand exactly how objects behave when pulled on by gravity. They have mathematical formulas to calculate orbits and the movement of objects that are attracting each other. This is how they can send a spacecraft to meet up with a particular comet at a particular moment in time, or predict exactly when Mars will be closest to Earth.

Deep Space 1 meets Asteroid Braille.
M100 spiral galaxy.

Now when astronomers look carefully at a galaxy, they can measure how fast the stars within it are moving. The motions of the stars are the result of the gravitational forces from all the other matter in the galaxy. But here is the key problem: When astronomers add up all the matter in all the stars and gas and dust visible with all different kinds of telescopes, it doesn't total nearly enough to explain the motions they observe. The stars are moving around much faster than they should be! In other words, all the matter we can see is not enough to produce the gravity that is pulling things around. This problem shows up over and over again almost wherever we look in the universe. Not only do stars in galaxies move around faster than expected, but galaxies within groups of galaxies do too. In all cases, there must be something else there, something we can't see, something dark.

This mismatch between what we see and what we know must be there may seem very mysterious, but it is not hard to imagine. You know that people can't float in mid-air, so if you saw what looked like a man doing just that, you would know right away that there must be wires holding him up, even if you couldn't see them.
Interacting galaxies.

The name scientists have given to the missing material is "dark matter." We can see the bright matter, like stars, but we know some other matter is there because of how it pulls on the bright matter. The black background of space that we tend to ignore when we enjoy the beautiful sights of the night sky really isn't as empty as you might think. As surprising as it might seem, there is more than 50 times more dark matter than bright matter in the universe.

But what exactly is the dark matter made of? Black holes and objects just a little too small to be stars (so they can't produce light of their own) make up some of it. But all of that adds up to less than one-fifth the total amount of dark matter that must be out there. We believe most of the dark matter is composed of new particles smaller than atoms that are different from anything scientists have ever detected and studied. It seems that the very essence of most of the matter in the universe is different from what you and I and rocks and trees and Earth and the Sun are made of.

Trying to understand dark matter is one of my favorite topics in science, so I'm glad I had the chance to shed some light on it for you.

[tommysize1]

i thought gravity could be affected by how fast, for example, a planet spins on its axis.

the faster it spins, the stronger the gravity.

i have no idea how they measure planets far out into the galaxy, or what measurements they take eg; weight, diameter, velocity
**** knows

but maybe an objects matter is just one part of the equation.

feel free to tell me otherwise im just guessing

[deltagirl]

an interesting discovery i made was the fact that the gravity on the moon is not one sixth the amount it is on earth..but about the same...This came about when mathmaticians decided to check up on newtons maths, which state the moon is one sixth the gravity of earths..even wenher von braun the captured nazi ss scientist who provided the know how for the the german V1s and V2s in the second world war and later became head of NASA rocket research, stated in 1974 that newtons maths were wrong...Several american book authors have highlighted this maths error, but still maintain that the moon landings took place!!..How can this possibly be when all of us have seen the NASA films of the astronauts "walking on the moon", supposedly jumping up and down easily "as if" they were in a gravity field one sixth that of the earths!!..Also the moon lander would have required much more fuel to break free of that extra gravity field..Which leads us inevitably to the fact that the moon landings never took place..no way..no how..lol

[tommysize1]

did they leave the american flag on the moon?
who's got a telescope!!!?

i reckon they landed on the moon,
i reckon they killed osama bin laden,
but
i reckon al queada was allowed to get away with 9/11
i also reckon JFK was allowed to be assassinated

i know theres alot of good reasons why all these conspiracies could be true..but they cant all be real...can they?

My brother said to me when I was A kid do you know whats faster than light I said what and he turned the light of and said darkness

maybe dark matter moves to fast to be able to see but what do I know

you know tommy I don't usually do conspiracy theory's but it looks so much like they planted asama binladen in Pakistan to make Pakistan look bad and right before a us election

[tommysize1]

i was on some nutjob site yesterday and they got as a headline
"china warns america that an attack on pakistan would be considered an act of aggression towards china"

anyway i think they got osama fair and square.
a few pakistani military/government officials probably agreed with osama's ideaolgy and helped him hide.
cant start a war over that

very interesting post about dark matter. the only thing i now about dark matter is that nibbler from futurama poo's it out

[Forger]

Wow, dark matter, dark energy, gravitational repulsion, exotic matter, the list goes on. This is amazing stuff, it explains everything from the red light shift theory to the big bang theory, infact it was made up to explain the unexplainable, to keep those untestable theories alive.
This stuff is so good it has a mass several times greater than everything in the universe..........but we cant see it. It has so much energy it is propelling everything in the universe outwards...............but we can't see it, it emmits no kind of visible radiation or light and can achieve reverse gravity. This invisible stuff is brilliant..................its so brilliant that it defies scientific laws. But the sad part is it doesn't exist in our galaxy...............that's convenient.

In order to believe it you have to call Albert Einstein a dimwit and his laws on relativity a child's musings.

[roobaby]

Blimey Forger! That was quite eloquent (in parts), lol! Of course, it is all underpinned by a belief that we all exist. I quite like Heidigger's idea that nothing is real, but we have such a fear of non existance that we create reality. I've tried that argument with the taxman, but it didn't work.