Solution:

If the image captured with your camera is too dark or too bright, the exposure value may not be optimal.

Exposure adjustments are a process in which the amount of light exposed to an image sensor is adjusted by the aperture value or the shutter speed to accommodate different levels of subject brightness. Correct exposure refers to optimal exposure for a subject, whereas underexposure is insufficient exposure and overexposure is excessive exposure.

If the Shooting mode of a Digital Single-Lens Reflex Camera or Interchangeable-Lens Compact System Camera is set to a mode other than <M>, the camera automatically sets the appropriate exposure value. However, depending on the shooting conditions or the subject, changing the exposure value manually is a better way to capture images that are closer to your intentions.
This is called exposure compensation. You can manually adjust the exposure by increasing or decreasing the exposure value that is automatically determined by your camera.

For further information for setting exposure compensation, please refer to the instruction manuals that came with your camera.