You're treating the absence of something as if it is, itself, something

It ain't.

Umbrakinesis and Photokinesis are the same thing. They may not be in the magical world of superheros and supervillain, but your tagged your question Science-Based. Darkness is the absence of light.¹ The only way you get it is by manipulating the light.

Why is this important? Because the contents of an empty glass could be described as "the absence of water." And I'm hard-pressed to believe that someone can manipulate what's in an empty glass and call it the ability to manipulate the absence of water. Kinda reminds me of Dwayne Johnson's line from Jungle Cruise. "The back side of water! ... It's totally different...."

So, scientifically, an "umbrakinetic" is a photokinetic who prefers dark rooms. But it's all the same superpower. The ability to manipulate light.

And how do you do that scientifically? Well... with gravity. Or by manipulating the atmosphere to create lenses. Or by manipulating water to create really dense clouds. Or, if you really want to channel your inner Einstein, give your superhero the ability to convert energy to mass such that the photons drop to the ground like so many grains of sand.

That might actually make for an interesting story. Your umbrakinetic creates darkness by converting light to mass, resulting in a substantial storm of "stuff" flying about like a sandstorm. Really fine "stuff." I'd probably be allergic to it. That'd be dark on several levels.

_{¹ Technically, darkness, which is something perceived by critters like humans with eyeballs, is the absence of visible electromagnetic energy. But the simplification works for the answer.}

You're treating the absence of something as if it is, itself, something

It ain't.

Umbrakinesis and Photokinesis are the same thing. They may not be in the magical world of superheros and supervillains, but you tagged your question Science-Based. Darkness is the absence of light.¹ The only way you get it is by manipulating the light.

Why is this important? Because the contents of an empty glass could be described as "the absence of water." And I'm hard-pressed to believe that someone can manipulate what's in an empty glass and call it the ability to manipulate the absence of water. Kinda reminds me of Dwayne Johnson's line from Jungle Cruise. "The back side of water! ... It's totally different...."

So, scientifically, an "umbrakinetic" is a photokinetic who prefers dark rooms. But it's all the same superpower. The ability to manipulate light.

And how do you do that scientifically?² Well... with gravity. Or by manipulating the atmosphere to create lenses. Or by manipulating water to create really dense clouds. Or by making everything in the area that's "dark" totally light absorbent (entirely non-reflective). That could cause some heat problems.... Or, if you really want to channel your inner Einstein, give your superhero the ability to convert energy to mass such that the photons drop to the ground like so many grains of sand.

That might actually make for an interesting story. Your umbrakinetic creates darkness by converting light to mass, resulting in a substantial storm of "stuff" flying about like a sandstorm. Really fine "stuff." I'd probably be allergic to it. That'd be dark on several levels.

_{¹ Technically, darkness, which is something perceived by critters like humans with eyeballs, is the absence of visible electromagnetic energy. But the simplification works for the answer.}

_{² What you're about to discover is that you can manipulate a lot of things other than light, all of which affect light. In other words, while it's certainly true that photokenisis is the opposite of umbrakenisis, the manipulation of light, itself, isn't actually the only path you can take to get to the "creation of darkness."}