  SKA is a new technology radio-telescope array, about two orders of magnitude
more sensitive and rapid in sky surveys than present instruments. It will probe
the dark age of the universe, just afer recombination, and during the epoch of
reionisation (z=6-15); it will be the unique instrument to map the atomic gas
in high redshift galaxies, and determine the amount and distribution of dark
matter in the early universe. Not only it will detect and measure the redshifts
of billions of galaxies up to z=2, but also it will discover and monitor around
20 000 pulsars in our Milky Way. The timing of pulsars will trace the
stretching of space, able to detect gravitational waves. Binary pulsars will
help to test gravity in strong fields, and probe general relativity. These
exciting perspectives will become real beyond 2020.
