  This paper is two-fold. At first we will discuss the generation of source
terms in the Einstein-Hilbert action by using (topologically complicated)
compact 3-manifolds. There is a large class of compact 3-manifolds with
boundary: a torus given as the complement of a (thickened) knot admitting a
hyperbolic geometry, denoted as hyperbolic knot complements in the following.
We will discuss the fermionic properties of this class of 3-manifolds, i.e. we
are able to identify a fermion with a hyperbolic knot complement. Secondly we
will construct a large class of space-times, the exotic $\mathbb{R}^{4}$,
containing this class of 3-manifolds naturally. We begin with a topological
trivial space, the $\mathbb{R}^{4}$, and change only the differential structure
to obtain many nontrivial 3-manifolds. It is known for a long time that exotic
$\mathbb{R}^{4}$'s generate extra sources of gravity (Brans conjecture) but
here we will analyze the structure of these source terms more carefully.
Finally we will state that adding a hyperbolic knot complement will result in
the appearance of a fermion as source term in the Einstein-Hilbert action.
