  Within the relativistic Brueckner-Hartree-Fock theory in the full Dirac
space, the tensor-force effects on infinite nuclear matter are elucidated by
subtracting the matrix elements of tensor forces from the realistic
nucleon-nucleon interaction. The tensor-force effects for the binding energy
per particle of symmetric nuclear matter (SNM) as well as the symmetry energy
are attractive and are more pronounced around the empirical saturation density,
while the tensor forces have little impact on the pure neutron matter. By
tuning the tensor-force strength, an infinite (negative) scattering length in
the spin-triplet channel is found. This locates the dilute SNM with only the
$^3S_1$-$^3D_1$ channel interaction at the unitary limit. Its ground-state
energy is found proportional to the energy of a free Fermi gas with a scaling
factor 0.38, revealing good universal properties. This work paves the way to
study the tensor-force effects in neutron stars as well as finite nuclei from
realistic nucleon-nucleon interactions, highlights the role of the tensor force
on the deviation of the nuclear physics to the unitary limit, and provides
valuable reference for studies of the four-component unitary Fermi gas.
