In the most recent episode of Boku No Hero Academia, we see that Tokoyami's quirk Dark Shadow loses control. What is the reason behind that and what exactly is Dark Shadow?
In my opinion dark shadow is like a physical form of tokoyami's thoughts which sometimes seems like a whole new personality in itself and this is his quirk, that he can give his rage a physical form. Seeing his friend bleeding by the attack of the villian with blades ,Tokoyami's anger goes out of control and so does Dark Shadow. Dark Shadow being uncontrollable in dark and controllable in light can be compared to vampire weakening in sunlight theory. Since both are fictional they can have similarities. We have also seen that Midoriya could not control his quirk at first so we may conclude that dark shadow is so powerful quirk that Tokoyami is still not able to control it properly.
I think they explained it well in that episode. When it's in the dark, dark shadow gets stronger but it's harder to control. The dark plus Tokoyami negative emotions contributed to dark shadow growing stronger and getting out of control. Light in the other hand, makes it weaker and easier to control. It's also mentioned in this wikia
According to Mezo, Fumikage's negative emotions (such as regret and indignation) probably intensify Dark Shadow, making it more unruly and possibly making Dark Shadow stronger as a result, albeit at the cost of Fumikage losing control; Fumikage himself stated that his anger causes him to lose control of Dark Shadow.