The image captured in DNG mode is a RAW image and it's not the actual representation or final product like the JPEG captures.
Often, our pictures don’t match what we actually saw when we were taking them. Editing is another chance to achieve that vision. A common format for taking pictures is JPEG, which is a compressed format that has limited editing options.
The moment you start editing the image you'll start to see the underexposed image getting better. However, if your captures are too dark or does not have enough light to let you edit, please share a screenshot with us so we may try and replicate it.