Normal space.

A quasi-parallel dimension, through which gravity propagates.  There is no matter, naturally, within t-space, but distance tracks on a 1-to-1 ratio with n-space.  Long distance travel is not instantaneous, but light years can be traveled in matters of hours.  Nearby solar systems might take a week or more.

Gravity Assisted drive core.  In other-wise matter-less t-space, gravity manipulation allows travel at many times the speed of light by creating a field of high gravitation in front of the ship and low gravitation in the back.

When the GA drive core is not in use as a propulsion system, it can be used to create an externalized deflection shell around the outside of the ship.  Direct kinetic force is, at best, slowed fractionally.  The less directly the force being applied (the less perpendicular), the more the grav shell is capable of redirecting or reducing the force of the impact.