It always surprises me when you make statements like this. What do you mean by 'known to be real' and what do you mean by theory? General relativity is a theory, is it not known to be real in your opinion? Is the Standard Model known to be real or 'just' a theory. I do not understand how you can make statements like this since I was under the impression that you had a degree in physics. The Casimir effect certainly seems real enough to me! Casimir predicted the effect from virtual particles in 1948 and experimentation in the 1970's showed the effect was real. So what's the problem?