Easily unless your definition of unfounded and theory is different to the ones in the dictionary.
Unfounded: without foundation; not based on fact, realistic considerations.
Theory: a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained. "Darwin's theory of evolution"
Vast majority of conspiracy theories are unfounded - They are also...
Groundless, baseless, unsubstantiated, unproven, unsupported, uncorroborated, untested, unconfirmed, unverified, unattested, unjustified, unwarranted, foundationless, ill-founded, without basis, without foundation, not backed up by evidence; speculative, conjectural, idle, vain, unsound, unreliable, questionable, misinformed, misguided, spurious, specious, fallacious, erroneous, fabricated, untrue, groundless, baseless, unsubstantiated, unproven, unsupported, uncorroborated, untested, unconfirmed, unverified, unattested, unjustified, unwarranted, foundationless, ill-founded, without basis, without foundation, not backed up by evidence; speculative, conjectural, idle, vain, unsound, unreliable, questionable, misinformed, misguided, spurious, specious, fallacious, erroneous, fabricated, untrue, trumped-up.
