I will use the Bohr model (together with the nature of light discussed in the last few posts) to predict the existence of "spectral lines," which will finally bring me back to dark matter by explaining exactly how we measure the speed of those rotating galaxies (see the Dark Matter Intro link if this is not familiar). Historically speaking, I'm presenting this material backwards, as the observation of spectral lines came first and the explanation came later, but I will proceed anyway.
Niels Bohr is in many ways the father of quantum physics, if not its prime mover. He came of age before the revolutionary wave of the 1920s, but almost all of the physicists involved in developing quantum mechanics (Heisenberg, Dirac, Pauli to name a few) spent some time at the Institute of Theoretical Physics he founded in 1921. His model of the atom was a precursor to all of the discoveries of quantum physics to follow.
So what is that model? Although it turns out not to be accurate, it's a pretty good start, and I would guess that the Bohr model is generally the picture most of us have in our minds for the atom today. Analogous to our picture of the solar system, the Bohr model imagines a dense, very small nucleus, surrounded by orbiting electrons (I took the picture from a website at Jefferson Lab).
By itself, that isn't all that interesting - the real theoretical interest of the Bohr atom was that the electrons are constrained to lie at specific orbits. In the solar system, the planets could theoretically lie at any radius - we could take the Earth, move it a little farther away from the sun, and it would still orbit contentedly (we might all be a bit colder, but the orbit would be fine). In Bohr's atom, an electron can't move to a slightly larger orbit; instead, it would have to jump to the next available orbit. The analogy is that the Earth can only be at our orbit or Mars' orbit, but nowhere in between.
To take this a step further, we can add energy considerations. The farther away from the nucleus, the more energy an electron has. Therefore, whenever an electron switches orbits, it gives up or takes in energy (depending on whether it's heading out or heading in). If it is only allowed to be in certain orbits, then the possible energy steps are discrete - it can only take in or give up a very specific amount of energy. For one more analogy, suppose my mom is on an elevator on the ground floor. If she goes up in the elevator, she can only get off on floors, she can't get off in between floors. And, as she goes up, she picks up a specific amount of energy (which she could give back if she were to jump out a window - she would be rather more regretful if she jumped out a 4th floor window as opposed to a 1st floor window because of all the energy she picked up by going up the elevator). The electrons in the orbit of the Bohr atom are like my mom on the elevator - they can't get off between floors and the amount of energy they can pick up or give out is discrete and fixed.
The very astute reader might see a similarity between this model and the photoelectric effect of Einstein from my post on the nature of light when light could carry a specific and discrete amount of energy depending on its frequency. In fact, this is exactly the same thing - how does the electron gain or give up energy? By absorbing or emitting photons (in the photoelectric effect, an electron absorbs enough energy to jump off the surface of the metal, or be "freed" from the orbit). This has significant results for observations of atoms as we'll see in the next post. For now, though, I'll put up one more diagram, similar to the one above, containing the positively charged nucleus at the center, and an electron that can be in one of three "energy levels" (or floors) by emitting or absorbing a photon (the wavy line). Also, for a little interactive demonstration of the same thing, try this.