Maybe he accidentally finds out when he is near some magnetic objects in Hogwarts. For example, he might notice that he can make the metal keys move towards him without using magic spells he already knew. It could be during a time when he is exploring an old, forgotten part of the castle where there are a lot of metal objects and strange magnetic fields.
Well, in such a fanfiction, he might control space through his magic wand. Maybe he uses a special spell that he discovers or invents, which allows him to manipulate the fabric of space, like creating portals or warping distances.
In some fanfictions, it could be through an accidental encounter. Maybe he was in a situation where metal objects started to react to him unexpectedly, like when he was near a bunch of keys and they suddenly started to move towards him. This made him realize that he had this strange new power.
Perhaps he has a strange dream where he's in a world made entirely of shadows and he can manipulate them freely. When he wakes up, he starts to experiment and finds out that he can actually do the same in the real world. He might start with small things like making the shadow of a quill move on his desk, and gradually discovers the full extent of his power.
Well, it might develop as a result of some new magic he encounters. There could be an ancient spell or a magical artifact that gives him the power to control sand. He then has to figure out how to use it. He may start with simple things like making sand swirl around his hand, and as he gets more used to it, he can do more complex things like creating sand barriers or even using the sand to communicate in some way.
Perhaps it's through an accidental encounter with a powerful magical plant. He tries to interact with it in a normal way but ends up influencing its growth and behavior. This unexpected event makes him aware of his unique power to control nature. For example, he might touch a withering plant and suddenly it springs back to life and starts to grow rapidly under his touch.
Some fanfiction might have characters control electricity through a special wand movement. For instance, a swish and flick combined with a specific incantation could direct electrical currents. This could be based on the existing magic system in Harry Potter where different wand movements and spells create different effects.
Well, he might gain the power to communicate with ancient magical creatures. Since he can control the void, it could open up new channels of magic, allowing him to speak to magical beings that were previously unreachable.
One challenge could be controlling the extent of his power. For example, he might accidentally cause a huge overgrowth of plants that start to overrun the Hogwarts grounds. It's difficult for him to fine - tune his ability at first.