Isn't this a pointless goal anyways since any spaceship we'd have capable of getting to any meaningful percentage of c would get destroyed by any small amounts of matter or gas it bumps into on the way.
Impact seems to have a timelined meaning, starting with destruction, then following up with conquering, and exploration coming somewhere before constructive applications at the far end.
As apes we just can't keep holding back till we can build houses or something with it, when we also could just throw that stone at something. Especially before someone else does :)
In the interstellar medium, matter is primarily in molecular form and reaches number densities of 10^12 molecules (mostly composed of) hydrogen, then helium, oxygen, nitrogen) per cubic meter (1 trillion molecules per cubic meter).[1]
So, what, about a picogram per cubic meter.
If your ship has a cross-sectional area of one square meter, and Alpha Centauri is 40,000,000,000,000 meters away [2] (and you thought it was a long way to the shop if you want a Chiko Roll), you’ll have to manage with 2.62^23 tonnes of mostly hydrogen in the way.
The interstellar medium is also about 1% dust, so about 2.6^21 tonnes of solid matter.
Someone check my mass maths.
1. https://en.m.wikipedia.org/wiki/Interstellar_medium
2. Google Search AI Overview
Dust is a concern but again the typical dust particle size would be about the power of a gun shot of a small caliber rifle. A problem to deal with but a rarer event still. Of course another challenge would be larger dust particles which while improbable are still possible which largely rules out humans in these craft. But that goes without saying since the acceleration to get to that speed would be unlikely something humans could withstand anyway.
Even interstellar/stellar wind would have enough molecules of gas to cause some crazy erosion/damage.