It's hard to argue why it was real when I have no real idea how one would go about claiming that it wasn't - if it was all in Cicero's head, where did Catiline suddenly get an army from and why would he have support from the Gauls? also how would Sallustius make any sense? Where did you hear this theory, what was the line of argument?

That being said, I do sometimes have the impression that Cicero had a paranoid streak. In his orations de lege agraria he claims to have uncovered another massive conspiracy that threatens the very existence of the republic (a mad ploy to use veteran colonies that are to be created in Campania to turn Capua into the new Rome) - earlier in the same year as the Catiline conspiracy. But even a broken clock is right twice a day and I don't see how Cicero could have been wrong about Catiline.