Cosmic Strings

[OmegaGuy]

Is there any actual scientific basis for the existence of these things? Has one ever been observed? Would it be possible (even theoretically) to create one in a laboratory?

[Lord of the Abyss]

Is there any actual scientific basis for the existence of these things?

Theoretical only. And many present theories hold them as unlikely to exist in the observable universe.

Has one ever been observed?

No, although they are theoretically observable by gravitational lensing; they would bend light in a way that other dense objects, like a black hole, don't.

Would it be possible (even theoretically) to create one in a laboratory?

Not that I know of; if it could be done, it would take a hugely advanced and powerful civilization. We're talking about a spacetime defect, not an object made of matter.