Two dark clouds on the spacetime horizon

(i) Investigations of stellar systems known as wide binaries suggest that Newton’s law of gravitation breaks down at low accelerations. This can be explained by proposing a fifth force with [Formula: see text] gauge symmetry, whose corresponding gauge boson is the sought for dark matter candidate. (ii) Relativistic causality (i.e. no faster than light signalling) permits nonlocal correlations stronger than those permitted by quantum mechanics. Can this be explained if our universe possesses a second 4D spacetime with a signature opposite to ours? (iii) Such a fifth force, and a second spacetime, arise naturally in a recently proposed theory of unification based on an [Formula: see text] symmetry. Our spacetime arises from breaking of an [Formula: see text] symmetry, and the second spacetime arises from broken electroweak symmetry.