When we think about our galaxy, most people think about the stars in the sky, the grand, sweeping spiral arms, the disk-like plane of our Milky Way filled with dust, and the bulge in the galactic center. All of this combines to make up our run-of-the-mill home, complete with some 400 billion stars not so different from our own. And our Milky Way, visible from anyplace on Earth during a dark, moonless night, is just one of hundreds of billions of galaxies similar to it in our Universe.
Yet this one is not only our own, it contains much more than what’s visible to us. In particular, the galaxy has a huge massive halo beyond the disk. It isn’t just full of dark matter, either, but various incarnations of normal matter, including more than a hundred globular clusters (collections of hundreds of thousands of stars, all bound together within just a few tens of light years) and — very importantly — gigantic clouds of molecular gas, moving at high speeds throughout the outskirts of our galaxy. These clouds can collapse and form stars, they can pass through the plane of our galaxy and trigger new episodes of star formation, or they can gravitationally interact with other masses, including:
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infalling dwarf galaxies or tidal debris,
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globular clusters,
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other molecular clouds,
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or passing clumps of (normal or dark) matter.