Double network hydrogels (DN-gels) are the toughest of crosslinked polymer networks which contain 90% water by volume. The fracture toughness of a highly swollen but brittle polyelectrolyte network increases by an order-of-magnitude from the in situ polymerization of neutral, linear chains. The order-of-magnitude increase in fracture toughness is intriguing and has not been previously observed in other conventional interpenetrating polymer networks. Here, we present insights into the structure and dynamics of DN-gels studied using recent neutron scattering measurements. The scattering from individual components within the DN-gels was obtained by using a deuterium-labeled monomer in conjunction with contrast-matched water. The highlights of our results are: (i) The two components are not isolated and instead form a stable associative complex in water, and (ii) Compositional fluctuations, probably associated with complex formation, correlated over large length scales (≈ 1.5 um) stabilize the structure of DN-gels under deformation. The possible role of the observed complexation in the toughening mechanism will be discussed based on the scattering data and other supporting measurements.