"The Dark Matter Dragon In My Garage" with apologies to Carl Sagan
"A fire-breathing dragon lives in my garage."
Suppose I seriously make such an assertion to you. Surely you'd want to check it out, see for yourself. There have been innumerable stories of dragons over the centuries, but no real evidence. What an opportunity!
"Show me", you say, and I lead you to my garage. You look inside and see a ladder, empty paint cans, an old tricycle - but no dragon
"Where's the dragon", you ask.
"Oh, she's right here", I reply, waving vaguely. "I neglected to mention that she's an invisible dragon".
You propose spreading flour on the floor of the garage to capture the dragon's footprints. "Good idea", I say, "but this dragon floats in the air". Then you'll use an infrared sensor to detect the invisible fire. "Good idea, but the invisible fire is also heatless", I say. You'll spray-paint the dragon and make her visible. "Good idea, except she's an incorporeal (bodyless) dragon and the paint won't stick!"
And so on. I counter every physical test you propose with a special explanation of why it won't work.
Now what is the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all? If there's no way to disprove my contention, no conceivable experiment that would count against it, what does it mean to say that my dragon exists? You're inability to invalidate my hypothesis is not at all the same thing as proving it true. Claims that cannot be tested, assertions immune to disproof are veridically worthless, whatever value they may have in inspiring us or in exciting our sense of wonder. What I'm asking you to do comes down to believing, in the absence of evidence, on my say-so.
But then I say, "Well, that's because the dragon is made of Dark Matter." And then you walk away satisfied that there really is a dragon in my garage, because apparently, scientists nowadays will believe anything, as long as it's told to them by other scientists. Witness Climategate."