Dark Matter - or - things I think about.

Our galaxy is about 100,000 light years across and it takes our sun about 240 million years to complete an orbit around the galactic center.  It's also about 20,000 light years THICK, with it's mass, apparently, concentrated in a thin central plane.  Our galaxy is made of stuff we can see, touch and feel.  And stuff we can't.

There's this stuff out there we know exists because we can see the effects of its gravity.  But that's all we can see.  It is, for all practical intents and purposes, invisible.  It's called Dark Matter, a poor choice of names, for sure, because it gives the wrong impression of what it is and does.  It's NOT DARK.  It's invisible.  We see right through it, it flows right through us without effect, and yet, without it our galaxy would fly to pieces.

If you compare the orbital speeds of Mercury, Venus, Earth and Mars, you'll notice a trend.  The farther from the Sun you are, the slower the orbital speed around the Sun.

Mercury - 47.87 km/s
Venus     - 35.02 km/s
Earth      - 29.78 km/s
Mars      - 24.007 km/s

This decline in orbital speed the farther from the center you are does not hold on a galactic scale, however.  It was noticed that the stars of the galaxy orbit at constant speeds, no matter what their position within the galaxy.  A star at the outer edge of the galaxy was orbiting at the same speed as stars closer in.  Something we couldn't see was holding the galaxy together.  The stars at the outer edge of the galaxy are orbiting so fast they should just fly off into intergalactic space.  And yet, they don't.

Our galaxy lives in a huge bubble of invisible matter whose gravity serves to bind the galactic pieces together.  This matter is over, under, around and through everything and, it is now thought, there is a concentration of this invisible stuff that lies along the galactic plane, just like the stuff of the galaxy that we can see.

So.  I told you all that to tell you this:

The Sun, our Sun, oscillates up and down across the galactic plane every 70 million years or so.  So, every 35 million years, our solar system crosses the plane of the galaxy, and some now think that this is when elements of the solar system are perturbed to the point they fall in toward the Sun. And maybe impact the Earth.  And cause extinctions.  But the real question is, what stops the Sun from just flying off into Inter-Galactic space?  It has been proposed that the concentration of Dark Matter along the galactic plane keeps the Sun from flying off - that the extra gravity of this invisible matter keeps us home, within that 20,000 light year thickness of the galactic plane.

It's an unfortunate term, Dark Matter.  It's really so much more.  And so much more important than its name implies.


Ndinombethe.  Ubuntu.