Existence of inertial frames

Question

Existence of inertial frames is an empirical fact.

What observations conclude that the existence of inertial frames is an empirical fact?

Answer 1

Since gravity extends through the entire universe with varying strength and thus causing tidal effects everywhere, there are no truly inertial extended frames in our universe.

An empirically inertial frame would be limited by your empirical measurement methods. You can very quickly see that a map coordinate on the surface of Earth is not an inertial frame by releasing an object and noticing that it seems to accelerate under some pseudoforce - in actuality of course it only follows a geodesic while the surface is accelerated toward it by a normal force from below. On the International Space Station tidal effects and atmospheric drag cause some acceleration but the effects are probably too small to measure with a yardstick and stopwatch. In deep space - if we could reach one of the great voids between filaments of galactic superclusters - even our best measurement devices may not be able to detect tidal acceleration. Beyond the problem of our tools, a "local frame" in physics can be defined arbitrarily small, to the point that inertial measurement is overrun with errors in measurement due to quantum uncertainty.

So to sum up: there are pointlike true inertial frames and extended "empirical" inertial frames actually in existence in our universe.