We argue that part of 'dark matter' is not made of matter, but of the singular world lines and world surfaces in the solutions of Einstein's vacuum field equation Gμν = 0. Their Einstein–Hilbert action governs, in a slightly modified form, also their quantum fluctuations in a partition function formed from a sum over all line and surface configurations. For world surfaces, the Einstein–Hilbert action coincides with that of closed bosonic 'strings' in four spacetime dimensions, which appear here in a new physical context.
