A common dead-horse concern dragged out for flogging by interstellar travel sceptics is the effect of running into small masses between the stars. Like all such discussions, without quantities to pin the arguments to, they remain nebulous.
[2111.02180] A radiation transfer model for the Milky Way. II The global properties and large scale structure
The above paper outlines the major components of the Milky Way – about 50 billion solar masses of stars, 5 billion of gas, 50 million of interstellar dust. It works out the scale heights of the components – thus most mass is concentrated around the Galactic Disk, falling off exponentially the further out one goes.
So lets assume all the dust is in the form of deadly pebbles, instead of micron sized specks. Sand grains, 1 mm on an edge, massing 2 milligams each. At 0.5 c they possess a kinetic energy kick equivalent to 6 tons of TNT (25 GJ).
50 million Solar Masses is a lot of dust – about 5E+43 grains of sand – spread through the Galaxy. Imagine it’s all concentrated near the Galactic Disk. So the Galaxy is about 60,000 ly in radius and the Disk about 200 ly high (and low) – a total volume of 4.5 trillion cubic light years. So about 1E+31 sand grains per cubic light-year. Sounds crowded! If we imagine a cubic light-year divided up into one metre squares each a light year long, there’s 9E+31 such metre wide track-ways through each cubic light-year. Thus each square metre of a starship would each encounter a sand-grain perhaps once every 8 light-years.
While that sounds deadly, it assumes we let bare starship to encounter the dust, instead of a series of precursor shields to blast the dust into plasma and divert it via magnetic fields.
In the real Galaxy the dust peaks in size at about 2.5 microns and thus packs a lot less punch per dust speck. There’s more of them, but the total flux is spread out over time.
The flux from the gas, which is 90% hydrogen by number, I’ll leave as an exercise for the reader.