[vee-kay]

Speed of light is a constant and law only in the 5% of the Universe that's made of Matter and Energy.

Light is energy, it is a form of electromagnetic radiation, that travels in waves and consists of particles called photons. While photons do not have mass, they carry energy and momentum, which allows them to interact with matter.

But remaining 95% of the Universe is made of Dark Matter and Dark Energy, so if aliens are made of dark matter and use dark energy technology, their laws of physics may be different, and they could be capable of interstellar travel because they may not be limited by light's speed. Their dark matter and dark energy based technology may be incomprehensible or even irreproducible by us humans.

Scientists are still not sure what Dark Matter and Dark Energy are, so 95% of the Universe is still a big question mark to us all.

[int_19h]

The very existence of dark matter is postulated by us based on those very laws of physics that include the speed-of-light limit as a foundational axiom. If general relativity is not really true, then there's no basis to infer the existence of dark matter from our observations.

But it could also be something like https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics.