Mechanics of moving and acting in a completely dark environment

Are there any mechanical consequences in the game for characters without darkvision when moving and taking actions in a completely dark environment (such as a pitch black forest)?

Are there penalties for moving in the dark when you cannot see?

Is it, for example, possible to use a weapon to shoot at a character standing in light if you are in darkness yourself, or would you suffer from not having light to see the path you are walking, load your weapon etc.?