And some think they now know the cause:
The Solar System floats in the middle of a peculiarly empty region of space.
This region of low-density, high-temperature plasma, about 1,000 light-years across, is surrounded by a shell of cooler, denser neutral gas and dust. It’s called the Local Bubble, and precisely how and why it came to exist, with the Solar System floating in the middle, has been a challenge to explain.
Michelle Starr, “The Solar System Exists Inside a Giant, Mysterious Void, And We Finally Know Why” at ScienceAlert (January 12, 2022)
The researchers think that star formation happens mainly at the perimeter (conveniently, one might add):
According to the researchers, this suggests that the Milky Way is likely full of similar bubbles, since the likelihood of this happening is very low if the bubbles are rare. The idea evokes a Milky Way structured similarly to a sea sponge, or perhaps a flattened wheel of Swiss cheese.
Michelle Starr, “The Solar System Exists Inside a Giant, Mysterious Void, And We Finally Know Why” at ScienceAlert (January 12, 2022)
Expected later research should shed more light.