Astrophysicists believe that our galaxy must be filled with more dark matter than ordinary matter. Now astronomers say they can find no evidence of dark matter’s gravitational influence on the planets. What gives?
Astronomers have a problem. Whenever they study the large scale structure of the universe, it soon becomes clear that the amount of visible matter cannot possibly generate enough gravity to hold together the structures they can see. Things like galaxy clusters and even galaxies themselves ought to fly apart given the amount of ordinary matter they contain.
Something else must be holding these things together. So astronomers have dreamt up the idea of dark matter—mysterious, invisible and non-interacting stuff that fills the universe, generating the gravity necessary to hold everything together.
This isn’t a small problem requiring a tiny amount of extra mass. The problem is huge. According to the latest picture of the large-scale structure of the Universe from the Planck space mission, ordinary visible matter makes up just 5 per cent of the total mass/energy of the Universe whereas dark matter makes up 27 per cent (the rest is the even more mysterious dark energy).
To make the numbers work, astrophysicists tell us that our galaxy ought to be at least 80 per cent dark matter.
That means our Solar System ought to be swimming in the stuff. Indeed, physicists have calculated that particles of dark matter ought to slam into each human on the planet at a rate of 100,000 times a year, as we saw last year.
But that raises an important question. If we’re ploughing through a thick sea of dark matter as astrophysicists suggest, why don’t we see evidence of it?
Most dark matter detectors work by looking for evidence of the collisions that dark matter must make with ordinary matter. A few of these experiments say they have found tentative evidence of these collisions.
But there is another way to look for dark matter—by its gravitational effects on the Solar System itself. If the Sun is surrounded by a thick soup of dark matter, we ought to be able to see its gravitational influence on the orbits of the planets, moons and asteroids.
Read more at: MIT Technology Review