There are a few atoms (or protons and electrons) in every cubic cm of space between the stars in the galaxy. These things have an average velocity, and an average deviation away from the average velocity relative to the local stars. If the spacecraft is moving through this medium at a speed large compared to the average thermal speed of the particles, then the particles will be hitting the leading side of the spacecraft, but only rarely the trailing side. The energy absorbed by the collisions will mostly be used to heat the spacecraft's hull. Note, you might need to put plates outside the spacecraft for this purpose so that the thermal conduction from the spacecraft doesn't overwhelm the temperature comparison.

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