  We investigate cosmological implications of an energy density contribution
arising by elastic dark matter self-interactions. Its scaling behaviour shows
that it can be the dominant energy contribution in the early universe.
Constraints from primordial nucleosynthesis give an upper limit on the
self-interaction strength which allows for the same strength as standard model
strong interactions. Furthermore we explore the cosmological consequences of an
early self-interaction dominated universe. Chemical dark matter decoupling
requires that self-interacting dark matter particles are rather light (keV
range) but we find that super-weak inelastic interactions are predicted by
strong elastic dark matter self-interactions. Assuming a second, collisionless
cold dark matter component, its natural decoupling scale exceeds the weak scale
and is in accord with the electron and positron excess observed by PAMELA and
Fermi-LAT. Structure formation analysis reveals a linear growing solution
during self-interaction domination, enhancing structures up to ~ 10^(-3) solar
masses long before the formation of the first stars.

Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology