I've been starting to shoot some live music recently and have been doing an okay job and see an improvement with each time I shoot. However recently I was shooting and all of my photos have come out very dark, too dark to use really. I was shooting in manual with 1/100 shutter speed aperture as wide as possible and a 6400 iso. Any advice is welcome. Thanks in advance.
Photography in a theatrical setting is one of the most challenging forms there is, both in terms of pushing the equipment you use to the absolute edge of their capabilities and in terms of requiring every bit of skill and experience you might have as the photographer.
Photography is the art of capturing light. Most concert/theatrical settings don't offer much light to capture and what light there is to capture is changing rapidly and the subjects are usually very animated.
The traditional solution to not much light (longer shutter speed using a tripod to hold the camera still) doesn't work because nobody on stage stands still for 10-15 seconds while you take a picture.
Manual exposure with Evaluative metering at about -1. EOS 5D Mark II + EF 70-200mm f/2.8 L IS II, 200mm, ISO 1600, f/2.8 (notice only the front line is in sharp focus), 1/200 second. I'm right on the edge of motion blur at 200mm and 1/200 second even when timing the kick at its apex. But the faces are sharp and that's what counts the most.
The traditional solution to capturing motion (faster shutter speeds) doesn't usually work because there often isn't enough light to capture a good image on a small sensor with a narrow aperture.
Under dim red and blue LED lights: Canon 5D Mark III + EF 50mm f/1.4, ISO 5000, f/2.8, 1/80 second and a LOT of post-processing.
In the end you have to balance the two as best you can AND use gear that allows you to capture as much of the scarce light that is present in the scene in as fast a time as possible. That means fast lenses (wide apertures), larger sensors, and cameras that are highly responsive (fast handling and AF). It also means you must use impeccable timing when you release the shutter. If a performer is jumping you can get by with a slower shutter time if you catch the performer in the instant they stop going up and start going back down. If they're running back and forth across the stage, catch them in the instant when they transition from moving right to moving left or vice versa.
The accepted answer to this question gives a general overview with more than a few links at the end to other questions here at Photography.SE that deal with individual aspects of concert/theatrical photography.
What was your aperture setting? "Wide as possible" is not an f/stop and is far too vague of a descriptor.
Simple answer is that you underexposed your shots. Most DSLR and mirrorless cameras have an exposure index scale that looks something like this: -2,-1,0,1,2. There will be a free-flowing dot that moves left and right indicating to you how bright the metered area is going to be. It helps to use spot metering so that you can know exactly which area is being read and how bright it'll be. If you had done this, or at least paid attention to the dot in the exposure scale, you would have seen that the dot was in the negative for all of your shots.
You need to first understand how to meter with your camera. You need to understand what the exposure index scale is telling you. How did you come to the conclusion that 1/100 at some unknown wide-as-possible aperture with ISO 6400 was the correct exposure? Was the dot in the middle of the scale?