  Theories of gravity coupled to forms and dilatons may admit as solutions zero
binding energy configurations of intersecting closed extremal branes. In such
configurations, some branes may open on host closed branes. Properties of
extremal branes reveal symmetries of the underlying theory which are well known
in M-theory but transcend supersymmetry. From these properties it is possible
to reconstruct all actions, comprising in particular pure gravity in D
dimensions, the bosonic effective actions of M-theory and of the bosonic
string, which upon dimensional reduction to three dimensions are invariant
under the maximally non-compact simple simply laced Lie groups G. Moreover the
features of extremal branes suggest the existence of a much larger symmetry,
namely the `very-extended' Kac-Moody algebras G+++. This motivates the
construction of explicit non-linear realisations of all simple G+++, which
hopefully contain new degrees of freedom such as those encountered in string
theories. They are defined without a priori reference to space-time and are
proposed as substitutes for original field theoretic models of gravity, forms
and dilatons. From the G+++ invariant theories, all algebraic properties of
extremal branes are recovered from exact solutions, and there are indications
that space-time is hidden in the infinite symmetry structure. The
transformation properties of the exact solutions, which possibly induce new
solutions foreign to conventional theories, put into evidence the general
group-theoretical origin of `dualities' for all G+++. These dualities
apparently do not require an underlying string theory.
