Preamble: I don't want to say that one type of background is always better than another, otherwise you might say that I'm not flexible enough. So the answer is, it depends on the context. For example, in a darkened room it would be better to use a dark background. In a room with lights on, a light background may be more appropriate.
However, if we have to choose one, I would say that a dark background is a safer choice. The main reason behind is that a dark background is more comfortable for the audience. Light backgrounds – especially white – release a fair amount of light that can be uncomfortable for the eyes. This is especially true when you present in darkened rooms, typical of TEDx events and seminars. In these situations, our eyes get easily tired and therefore it’s better to use a dark background.
Also, consider this: most slides suck. And most slides have a white background. Of course that's not the reason why slides suck, but there tends to be an association in people’s minds between white backgrounds and poor slides. If you start a presentation with a slide on a white background, the audience automatically associate your presentation with all the presentations they’ve seen in the past which probably weren’t pleasant experiences. If you start with a slide on a dark background, you stand out. This is not enough to make a good presentation, there’s much more to it. However, it’s a first sign that shows the audience that your presentation will be different.
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