They are present as long as there is dark matter flowing through the earth. In essence the gravity of the earth just changes the density pattern of the stream o

They are present as long as there is dark matter flowing through the earth. In essence the gravity of the earth just changes the density pattern of the stream of dark matter particles the same way a lens focuses photons in the focal point [1]. So the interesting question is how persistent the stream of dark matter particles is. Also the earth is moving all the time around the sun and the centre of the galaxy and this alone should create a continuous stream because the earth moves through the dark matter background. So the even more interesting question is how dark matter is distributed - is there a more or less uniform distribution or is it patchy and the earth alternates between moving through essentially dark matter free regions and regions filled with dark matter. Unfortunately I have no answer for that. [1] https://en.wikipedia.org/wiki/Caustic_(optics) https://en.wikipedia.org/wiki/Caustic_(optics)