Every DSLR has a display feature called histograms. It looks like a graph with random data that any new photographer don’t know what it actually is. And maybe, won’t even care. Here’s a peek, it’s a reader that records how much light (how bright or dark) a picture is .
You should know that every LCD screen – camera, TV, or digital frame – could display different brightness level of a particular picture. What you see bright in this screen might not the the same in that screen.
This is why histogram comes in. It is the standard reader to record how bright a pic. A sample, you had an underexposed pic. So this should looks quite dark as shown in histogram. Now in your camera, it looks dark as expected, but it is quite bright in your computer screen. Chances are, your computer screen are set a little brighter than normal.