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The density of the interstellar medium varies hugely, so the specific problem of Sol-to-Proxima-Centauri travel is different from the general problem of interstellar travel. According to WP: In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of $10^6$ molecules per $cm^3$ (1 million molecules per $cm^3$). ...
Based on this answer: $$E \approx \frac{1}{2} m v^2 = \frac{1}{2} m c^2 \left(\frac{v}{c}\right)^2$$ The mass of a proton $m_P c^2$ is about 938 MeV, so if an innocent atom of hydrogen or bare proton in space were hit by a spacecraft at 0.1 c, in the spacecraft's frame it would look like a speedy proton. Of what energy? E = \frac{938}{2} 0.1^2 = 4.7 \text{ ...