But so far as we can tell, the theorists are taking this one seriously: From “Neutrons escaping to a parallel world?” (Physorg, June 15, 2012), we learn,
In a paper recently published in European Physical Journal C, researchers hypothesised the existence of mirror particles to explain the anomalous loss of neutrons observed experimentally. The existence of such mirror matter had been suggested in various scientific contexts some time ago, including the search for suitable dark matter candidates.
Berezhiani believes it could be interpreted in the light of a hypothetical parallel world consisting of mirror particles. Each neutron would have the ability to transition into its invisible mirror twin, and back, oscillating from one world to the other. The probability of such a transition happening was predicted to be sensitive to the presence of magnetic fields, and could therefore be detected experimentally.
This interpretation is subject to the condition that the earth possesses a mirror magnetic field on the order of 0.1 Gauss. Such a field could be induced by mirror particles floating around in the galaxy as dark matter. Hypothetically, the earth could capture the mirror matter via some feeble interactions between ordinary particles and those from parallel worlds.
More information: Z. Berezhiani, F. Nesti, Magnetic anomaly in UCN trapping: signal for neutron oscillations to parallel world? (2012), European Physical Journal C 72: 1974, DOI 10.1140/epjc/s10052-012-1974-5
And if that don’t rot yer sox, here’s an argument for parallel universes (2001) from the BBC:
Look, we’re allowed to do science fiction on Fri and Sat nite. Popcorn 9-11.