Why do most websites use a white background?
Apart from the ui being neat and simple, Is there any other reason why every major website uses a white background with very few colors?
This answer is more general, instead of just white I'll focus on light backgrounds.
There are two main factors that influence the fact that most websites use a light background, the first is the power of defaults and the second is readability.
Defaults and standards
HTML and CSS Styling in browsers defaults to white, most of the users, in this case the developers tend to stick with the default options. Just google "The power of defaults" to see what I mean.
There is also an evolutionary component about where the interface comes from and which objects in the real world it is trying to imitate. It could be really straightforward to think that given that the main use is to display information designers tried to emulate books which are light text over dark backgrounds. (I don't have evidence about this).
The second reason is that dark text on light backgrounds is better for readability, and this is a very controversial topic.
Text readability depends on a bunch of factors: mainly contrast, but also font size, color and lighting conditions.
Contrast and color choices
Assuming that the contrast is right and equivalent (W3C Accessibility Guidelines on contrast) for both light-on-dark and dark-on-light, it seems that for readability purposes the best thing is dark text over light backgrounds:
You should avoid using white text on a dark background when displaying paragraph text to make it easier from them to read. Forcing users to fixate on the white text for a long time can strain the user’s eyes. This is because white stimulates all three types of color sensitive visual receptors in the human eye in nearly equal amounts.
White also reflects all wavelengths of light. Because the words and letters in paragraph text are compact and close together, when white text reflects light, the reflected light scatters and runs into neighboring words and letters.
Here is also another source that collected some studies on the topic:
However, most studies have shown that dark characters on a light background are superior to light characters on a dark background (when the refresh rate is fairly high). For example, Bauer and Cavonius (1980) found that participants were 26% more accurate in reading text when they read it with dark characters on a light background. Moreover, a survey by Scharff, et al. (1996) revealed that the color combination perceived as being most readable is the traditional black text on white background.
And here is another study on the topic: Letter identification performance is better for negative contrast than positive contrast.
However, some people prefer the opposite (although I didn't find any evidence), ie. light text over dark background when they're reading in dark rooms, in fact, most ebook readers have the two options:
This software engineer prefers light text on dark background as a way to relieve eyestrain over long hours of programming. I sometimes refer to my job as 'staring at a lightbulb'. And that's why I disagree with the two contrasting text boxes you posted above. They refer to what happens in *print*. But on the computer screen, white is the emittance of energy, not the reflectance of it.
I agree with you but the debate is long, there is a long thread here http://www.stackprinter.com/export?service=stackoverflow&question=498698&printer=false&linktohome=true and although most programmers seem to agree with you I couldn't find evidence. I don't know if is a problem of research bias or something similar.. .
I am a programmer too, and I vote for the same as psoft. As for evidence: there is scientific evidence for that, concerning the amount of photons that hits the eye and their different characteristics while being reflected off a print or emitted by a lightsource inside a computer screen. If you just google for that, there'll be more concrete explanations. Also, there are standards on how a workplace should be lightened, its luminance and contrast with general background. These differ for paper and screen. Again, no prooflinks, but it easy to find in the corresponding documents.