Scientific presentations - black or white?
Conferences tend to take place indoors in dimly-lit rooms.  Dimly-lit rooms are a good place to fall asleep.  Using a white background helps avoid this.  Also, dark text/lines on a white background are easier to read (I've seen more than one study showing this).

If you are going to be showing a lot of astronomy or fluorescence biology images, where the image is mostly black with some interesting colorful things in it, you probably want a black or dark background for at least those slides; at some point you should just switch over.  Also, if it is essential to understanding for the viewer to discriminate many shades of color, it's easier with a black background because you can use a wide variety of discriminable pastel colors that would be washed out to invisibility on a white background. You can't even use saturated yellow or cyan on a white background and expect it to be seen.

But for most scientific presentations there are good reasons for a light colored background.