It's believed that dark matter is what helps keep the galaxies we see stay in their formation. Although dark matter is still just a theory, it's a rather solid one. If your looking for more information regarding dark matter, I highly suggest checking out the "The Universe", which is a tv series on the History channel. The episode dealing with dark matter is on season 2, episode 6.

I could read about space AND superman all day long. This was a great article I actually saw a history channel special about the same thing.

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Spiral Galaxies, like this one, basically spin from their beginning. Astrophysicists agree that most spiral galaxies have supermassive black holes at their centers, the gravity of which is part, but apparently not all, of what keeps them in their lovely shape, kind of like the skirt on a flamenco dancer as she spins around and 'round.

As Rakashu states, the remaining matter, or skirt in my analogy, is made up of stars, gas, dust and possibly something more (possibly a lot more), something known as "dark matter" because we cannot see or detect it any way except by how it may apparently affect the spinning movement of the galaxies.

The theory which leads to the supposition of "dark matter" as the stuff which holds these galaxies together has never sat well with me. In the past, I've posted comments and questions regarding the possible existence of larger than previously accounted for quantities of interstellar dust as the culprit, but here I'll take a different tack...

The motion which is unexpected within these galaxies is, as I understand it, the circular speed of the outer portions of the arms. According to Kepler's third law, because they are farther out, they should be going a lot more slowly around the gravitational center, or black hole, then they do. Or, putting it visually, as I understand it, the spiral should look a lot more "spirally", or should be wound up tighter, like the mainspring on a watch. Instead, there is the graceful sweep of the two main arms, with some intermediate minor structures in between, and these two arms, almost like the arms on my lovely Flamenco, trailing a pair of hand scarves, proceed to follow around and around, almost as if they are connected solidly, when, according to Kepler's law, they should trail slowly like the outer planets Neptune and Pluto do in comparison to the rapidly orbiting inner planets Earth, Venus, and the fastest, Mercury.

So what could cause this happen? Is there, as the theory suggests, a huge unseen "halo" of undetectable stuff? Or is there instead perhaps more gravitational interaction going on between the stars that make up the arms of the galaxy than we have previously realized? Even within our own solar system, the planets do not perfectly obey Kepler's 3rd law. They each gravitationally affect the others, nudging and dragging them, causing tiny perterbations in the expected orbits. That's how Neptune was first discovered. Astronomers carefully tracked the orbit of Uranus and found that it didn't quite fit the predicted path. With some very careful math calculations they deduced that another large planet orbiting beyond the orbit of Uranus could account for the differences and when they pointed their telescopes at the predicted location... ta-daaah! There it was!

So, my question is this, if gravitational perterbation can account for the anomolies in planetary orbits, has this been taken into account in analyzing the motion of these far off galaxies? Before we go proposing the existence of strange new unknown kinds of stuff to explain these motions, shouldn't we be absolutely clear about the gravitational interactions of relatively dense collections of star matter? or putting it simply, as in my analogy, couldn't it just be that the dancing black hole is swinging her own comparitively solid arms around?

Perhaps, when we know how gravity, and possibly gravity waves (if they even exist), really work, we'll get an answer to this question, but until then, I guess we're still in the "dark".

surely they've considered this. i can't imagine they would overlook such a simple explanation.

I just reread that and I realized that I hadn't been quite clear at the end there. I don't mean to incorrectly imply that the galactic arms are any where near as dense as the balck hole at the center, but that in comparison to the virtually empty space between them they are relatively dense and might be dense enough to hang together due to stronger gravitation than we currently expect. I hope that's more clear.

peace,

that's what I keep thinking, but in general, simpler explanations are usually better. I just don't understand why we need to invent something theoretical and mysterious like "dark matter" when we don't even really understand gravity.

good point! i'm all the time thinking of stuff that i wish i could present to a cosmologist or other physicist just wondering if they had thought of that before. you NEVER know when some un-physics-educated simpleton like me might come up with a concept that no one else considered because it might have been slightly outside their educated box. they might be so stuck in their ways that they don't think of something that we might. ya just never know!!! i wish i could be part of a "think tank of the masses" and just mull this stuff and present things!

and oh yeah, mikey, i've read many of your comments and always thought you were pretty bright. keep it up!

Thanks peace, I do what I can to contribute to the discussion...

I'm thinking the thick arms look similar to what the computer generated models show takes place after two galaxies collide. And it appears to me that this ones near the point when the two colliding galaxies are completely joined together.

Thanks, peace6, I too wish I could be part of a "think tank of the masses", as you so aptly put it. I don't have a lot of thought about colliding galaxies other than total amazement, but I could learn so much from such a thread.