Sunset photos are relatively easy to expose for because there is a lot of latitude or leeway in creating a "good" exposure. If you underexpose a bit (give too little exposure) then the photo will be dark; if you overexpose (give too much exposure) the photo will be too light. Because sunsets are so colorful and bold, however, it's likely that unless you really miss the mark, a wide range of exposures will look good.
One exception is when you include a very bright sun in the frame when you're setting the exposure. Because the sun is so bright (bright enough to light half the planet at a time), it will wreak havoc with exposure and cause your camera to think there is far more light in the scene than there actually is. Result: a grossly underexposed (dark) photo. One way to avoid that is to aim the lens away from the sun itself (actually exclude the sun from the composition) and meter from a bright area of sky without the sun. Then use your meter-lock function (usually this means holding the shutter release partially down) to lock in that reading. Now recompose and shoot and you should have an exposure based on the sky, but not the sun.
The problem is that sometimes there is no clear area of bright sky to meter from. When I was shooting this pretty sunset at Cypress Gardens in Florida, for example, I was surrounded by trees and didn't have a clear shot at open sky. Instead, I took advantage of the trees in the scene and simply hid the sun behind the center tree when I set my exposure. In fact, I found that once the sun was behind the tree, I was able to expose for the scene exactly as the composition shows. I got a perfect exposure with the camera in the Program (auto) mode.
So next time you're shooting a bright sunset and can't take a sky reading, just hide the sun behind something in your scene and your exposures should be perfect.
Beautiful San Francisco
10 hours ago