Invisible Matter and the Fate of the Universe

Contemplation of the fate of the universe, and of course the end of the world, reminds me of the story of a traveler in the Swiss Alps. After hiking high into the mountains he and his guide spent the night in a chalet. Early the next morning he was awakened by a resounding crash, followed by a deafening roar and several loud cracks. Jumping out of bed in fright he ran to his guide and yelled, "What is happening? Is the world coming to an end?" The guide calmly sat up in his bed.
"No … don't worry … it's just the rays of the sun. When they touch the snow on the peak of the mountain they cause it to hurl down into the valley. Then the rays warm the glacier, causing it to crack. It's not the end of the world, it's the dawn of a new day."
In the latter part of the book I will also be talking about the end of the world, and the fate of the universe. But, like the Swiss guide, I'd like to assure you that there is nothing to worry about: barring something unforeseen, no serious astronomical catastrophe will take place for millions of years. Still, the final fate of the universe is something we would like to know about, and understand. What will eventually happen to it? The key, it seems, is the amount of matter it contains. And, as you will see, part of this matter is invisible, or is at least hidden from us in some strange form. Scientists refer to it as dark matter.