From this page of the Baldurs Gate 3 guide you will learn how to illuminate the darkness that appears in some of the locations in the game's world.
Not all places in the game are well lit. On the one hand, it's a good thing, because if there's an enemy in the area then the darkness can help you sneak through and avoid detection. But if you want to search an area, that's a problem. However, the creators have implemented tools to help you solve the problem.
The most common of these is the torch. This item will be in the equipment practically from the beginning of your adventure. To work, a Torch has to be placed in a weapon slot and lit from any source of fire. If needed, a torch can act as a weapon.
The second option is to use magic. Wizards, clerics, and bards are in knowledge of spells that can generate light. Casting a spell like this will help you lighten the darkness.
If you manage to save Arabella during the quest, she will offer you a precious medallion that allows you to light up the darkness.
The last way to light up the darkness is to choose the right race. Tieflings, Elves and Half-Elves do not need any light source because they can see very well in all conditions.