  This article examines the foundation of the recently developed relativistic
variational formalism[1]. Our work is heavily based on [2, 27] which extends
this approach to the multi-fluid theory and examines its utility in
astrophysics and cosmology. Unlike the extension to the formalism mentioned
above, that looks at the general interaction between different types of matter,
we use the formalism to examine the interaction involving, ordinary matter,
dark matter (DM) and dark energy (DE). We focus on entrainment phenomena
involving the dark-sector constituents.
