The dark matter would not interact with our normal matter- it would flow through the planets without any interaction with the matter that makes up the planet… but it WOULD be affected by the planet’s gravitational field.

The planet would effectively act like a lens for dark matter- focusing it, concentrating it at a point in space ‘downstream’ of the planet- this concentration could be very significant- with Jupiter potentially concentrating dark matter by a factor of hundreds of billions over the background level…

Observations of clusters of galaxies show that they are individually moving so fast that the clusters should have dispersed long ago. There must be additional, invisible mass (dark matter) in the cluster that helps keep it from flying apart.

In both of these cases, we can calculate (using Newton’s Laws) how much dark matter must be there in order to explain these effects, which allows us to come up with estimates of what proportion of the matter in the universe is dark.

My own suspicion is that we haven’t really gotten to the bottom of this story. There is something else going on we haven’t stumbled on yet.

So for example, the rotation of a galaxy is determined by the total amount of matter it contains, although we can only see the luminous part of that matter in order to determine the rotation. Also, clusters of galaxies should fly apart, unless they contain much more matter than can actually be seen. That’s how dark matter was discovered in the first place, it was noticed that their gravitational interaction was NOT determined by Kepler’s and Newton’s laws. They seemed to have much more matter than was actually there.

Now, it isn’t absolutely certain there is such a thing as “dark matter”. Perhaps this gravitational discrepancy we see is caused by something else; perhaps the law of gravity operates differently over large distances, or maybe there is something else entirely going on we haven’t thought of yet. But it appears that today, the explanation that best fits the facts is that most of the matter in the universe is invisible, and it can only be detected by its gravitational effect on the visible matter (stars, gas, dust, etc).

If dark matter is indeed invisible matter, we still have no clear idea what it might be. Perhaps it is composed of Weakly Interactive Massive Particles (WIMPS) like neutrinos. Another suggestion is that it may be the gravitational effects of large amounts of ordinary matter lumps of a size that cannot be observed (too cold to give off any radiation) but too small to cast shadows, concentrated in the outer regions of galaxies, MACHOS (MAssive Compact Halo Objects). But nobody really knows, and both of those hypotheses fail to explain all the observations satisfactorily. All we know for sure is that the universe seems to contain a lot more matter than we can see, and as your article indicates, that matter interacts with visible matter gravitationally, in a predictable way.

Oh, and another thing. Dark Matter doesn’t seem to be connected in any way to Dark Energy. But that’s something else altogether.