Not a creationist here, but as a Christian I feel compelled to be open to many possibilities of the causes of the universe (but I don't believe the universe was created randomly out of nothing). I am going to address some of the points you bring, a little bit as a Christian, but also as a fan of astronomy.
But there's this pesky little thing you've heard of it--called Dark Matter.
Don't forget the larger portion of this concept, which is Dark Energy which theoretically stands at 73% of the make up of the universe (to a paltry 22% for Dark Matter). It would seem that even creationists are wrangling with this idea/concept. See creationwiki.com for some insight. http://creationwiki.org/Dark_energy
They seem to accept that the universe is getting bigger, and accelerating at the same time, and have some creation astronomers to address this (I didn't know there was such a thing as a 'creationist' astronomer!).
Why did God create it?
From a Christian POV, this is easy. IF this theoretical idea of Dark Matter/Energy holds up after further investigation and more is learned about over time, then mainstream science will accept this as truth. At that moment Creationists tend to go one of two ways: it either they will "discover" a way for it to fit into their world view, or deny it and arrive at a different theory.
Assuming there IS a God, why would he create Dark Matter? He sent his son to Earth to die for people that wouldn't even discover Dark Matter for 2000 years.
The same could be reasonably be said about all the recent scientific discoveries of the last 100 years. This assertion does not weaken Creationism. Maybe other things do, but not the argument that it took us 2000 years to discover Dark Matter. Sort of a non-point IMO.
We didn't even discover the marvel of planets and galaxies until we create telescopes.
The marvel of planets were well known thousands of years ago. All the great ancient civilizations (Babylonians, Greeks, Indians, Chinese, Egyptions) had observed the planets, had formed some scientific principals that laid down the foundation of astronomy, and had names for them. Being that the night time sky was 10x clearer and visible than it is today, it probably was a marvel for them. Better than Hi-Def televisions! Telescopes greatly extended that marvel to things beyond our naked sight. I would also add that as far as the notion of "marvel" goes, all of mankind, before the Industrial Revolution, had the marvel of the night time sky, being able to see the entire Milky Way Galaxy pass over you every night, with the only exception being if it were cloudy. I've seen it as described as taking a large paint brush dipped in the brightest yellow paint and swashing it against the biggest, blackest canvas you can find. Heck, unless you live in the Australian Outback, the Andes Mountains, or the middle of the ocean no one can see it the way we used to. Not such a marvel anymore.
So God clearly didn't create these things for us to marvel, because the Bible specifically says not to seek knowledge of contradictions (1 Timothy 6:20-21). That's what Science is. One giant contradiction to the Bible.
Your reference to Timothy has actually been interpretted quite a bit differently, specifically as an avoidance not of knowledge of the world or contradictions, but rather a warning about Gnosticism which was a heretical form of Christianity in the early Church and how, according to Gnosticism, there were multiple superhuman intermediaries between God and the world. The verse in Timothy probably reminds us that there is one intermediary (Jesus Christ). I don't think the verse really addresses what you suspect it does.
"Whoever loves discipline loves knowledge, but whoever hates reproof is stupid" Proverbs 12
But let's pretend that galaxies WERE created for man's marveled show the power of God... Why Dark Matter? Couldn't gravity have been made a little stronger? Or stars a little larger? I mean, while we're creating, why not just make everything work by design? Why do we need this invisible force of gravity? If there was a God and Genesis is true, then the solar system wouldn't need to exist within a galaxy at all. If we don't need galaxies, then we don't need Dark Matter.
But there it is. Dark Matter. The invisible.
I think statement is a bit of a stretch, even for a creationist. Creationists are not entirely blind to science are not stone throwing cave people... some have advanced science degrees, PhD's, and live in the academic world publish papers on their research. In fact I would suspect they have to be on top of every major scienctific break through and be knowledgabvle of all the current trends in science. Creationists do not fear science, rather hold that it proves they are correct. They speak to gravity, they speak to theories in physics, mathematics, biology etc...Now you may not agree with conclusions, but they have smart cookies (smarter than you and me). Here is a list of some creationist scientists and their field of study (this list shocked me). http://www.answersingenesis.org/home/area/bios/
For me, Dark Matter SCREAMS universal evolution. Galactic evolution. Without it, solar systems (this one in particular) may never coalesce.
For creationists, dark matter is still a theory, and they have come up with some alternate theories as to why there is an unknown reason for the gravitational discrepancy in the universe.... dark matter/energy currently fills this knowledge gap and can still be proved wrong. Not being a creationist I don't see the problem of DM or DE existing.
The universe would have exploded into nothing but a big ball of gas that would have never clustered into anything. But instead, it works as a catalyst. Like the rest of nature, it is its own creator.
I have an issue with this... how can the universe be it's own creator?
Anyhow, that's my take. Thoughts?