How to search for interstellar dust tracks:
First, learn how to focus the microscope. It's easy: just move your mouse along the blue bar just to the right of the microscope viewer. Don't hold down any mouse or keyboard buttons.
Next, find the surface of the aerogel. It is usually easy to recognize: it is generally covered in small imperfections that quickly go in and out of focus and you focus up and down. This particular field of view also has a large crack extending from top to bottom in the image, and a few smaller cracks in the right side of the image.
Next, focus just below the surface, and look for tracks. There is one in this focus movie.
To identify a track (this is really important) click on the deepest part of it that you can see, even if it is out of focus.
Move the mouse cursor across the blue bars to change focus.