Several extensions of General Relativity and high energy physics intend
to explain the accelerated expansion of the universe by including scalar
fields as extra degrees of freedom. At present, there is a broad
spectrum of theories that fulfill this requirement and at the same time
have no effects at very small scales, which makes the able to pass solar
system tests. In order to know if these scalar fields indeed exist, we
need to find predictions at intermediate scales in between those
involved in the expansion of the universe and the solar system (i.e.
galaxies and galaxy clusters scales). I will summarize some of the
properties of two particular families of scalar-tensor theories
(symmetron and chameleon models) and I will show why observations of the shape of dark matter halos could help to disentangle between them.