Why are sunspots dark?

Sunspots are a magnetic phenomenon on the Sun. You can think of them as small pores on the surface of the Sun where lines of magnetic force enter and exit. Sunspots always come in pairs like the north and south poles on a bar magnet. The strength of the sunspot magnetic fields are usually 1000 times as strong as the average solar magnetic field. Because magnetic fields can produce pressure, inside sunspots, the gas does not need to exert quite as much pressure as elsewhere on the Sun to insure that the total pressure across a sunspot is in equilibrium with the gas surrounding the sunspot. Since the cooler a gas is, the less pressure it exerts, this means that the gas inside a sunspot can be cooler than the gas in the rest of the solar surface and still, with the help of the sunspots magnetic field, remain in equilibrium.

The average gas temperature of the solar surface is about 6050 K, but inside a sunspot, the gas temperature is only 4200 K. The reason a sunspot appears dark is that the gas inside the spot where the magnetic field is strongest is only emitting about 1/4 as much light as from the rest of the solar surface. If you were to rip a sunspot out from the solar surface and put it in the night sky, it would appear as a bright, orange gas, not a dark void.