"Although the density of atoms in the interstellar medium is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles."

https://en.wikipedia.org/wiki/Interstellar_medium

Because the entire Milky Way is mostly at rest relative to itself.

If you are charging full speed ahead into its center, you are going against just about everything, including energetic particles no doubt coming the from the crowded center of the galaxy.