Dark Matter ... Beginners

I am reading about dark matter these days. Highly interesting ... First question ... what is dark matter. Simple ... its matter thats dark. Oops ... that sounded like a typical consultant ... or, Sir Humphrey (or are they pretty much the same thing?)! Well ... to make it simple ... Dark matter is stuff which is out there in the universe which we cant see. For some reason ... dont ask me why. The brains (read scientists) havent figured that out yet, or at least thats what i think.

Only thing i have been able to figure out yet is how they figured out its existence if they cant see it. Well ... its simple. We all know galaxies rotate around a centre. Now, the rotation speed of galaxies can be calculated using the Doppler effect. So, if the galaxy were like a disk, then one end of the rotating disc would be coming at you (blue shifted), and the other end would be going away (red shifted). Based on the extent of the spectral shift, they could calculate the rotational speed of the galaxy. Knowing this, the mass of the galaxy can be calculated using Newtonian mechanics. Now, the speed of rotation of galaxies is such that the stars along with their solar systems should probably get hurled out of the galaxy, given the calculated gravitational pull of the mass calculated for the galaxy by the Mass-Luminosity equation.

In other words, the calculated mass of the galaxy is not enough to keep these stars in place in the galaxy, and overcome the centrifugal force they would feel. Now, since the stars are actually in place in the galaxy, the only thing that can explain this is that there is mass in the galaxy which we cant see. Ergo, Dark Matter ...

According to what scientists say ... Mother Nature has hidden away 90% of the universe from us. Question is ... why? I dont know! Maybe someday soon, we will find out.