  In this paper, we have considered vector valued reproducing kernel Hilbert
spaces (RKHS) $\mathcal{H}$ of entire functions associated with operator valued
kernel functions. de Branges operators $\mathfrak{E}=(E_- , E_+)$ analogous to
de Branges matrices have been constructed with the help of pairs of Fredholm
operator valued entire functions on $\mathfrak{X}$, where $\mathfrak{X}$ is a
complex seperable Hilbert space. A few explicit examples of these de Branges
operators are also discussed. The newly defined RKHS
$\mathcal{B}(\mathfrak{E})$ based on the de Branges operator
$\mathfrak{E}=(E_-,E_+)$ has been characterized under some special
restrictions. The complete parametrizations and canonical descriptions of all
selfadjoint extensions of the closed, symmetric multiplication operator by the
independent variable have been given in terms of unitary operators between
ranges of reproducing kernels. A sampling formula for the de Branges spaces
$\mathcal{B}(\mathfrak{E})$ has been discussed. A particular class of entire
operators with infinite deficiency indices has been dealt with and shown that
they can be considered as the multiplication operator for a specific class of
these de Branges spaces. Finally, a brief discussion on the connection between
the characteristic function of a completely nonunitary contraction operator and
the de Branges spaces $\mathcal{B}(\mathfrak{E})$ has been given.
