What do you mean by darker? Darker than what?

Any photo can be darker than some other image of the same scene, just by adjusting the exposure level on the camera. An HDR image is usually made by combining several LDR images taken at various exposure levels. Until you view it, it is not darker or lighter - it is just data. If made correctly, the data indicates the actual luminence of every point without regard to artificial limits imposed by 8-bit images or traditional digital camera encodings (even 14 bits is not enough) or by the limitations of your display device. It is just data. Trying to look at that data directly on your monitor is meaningless and impossible, because it covers luminence levels far greater than your monitor can produce, and because your monitor is not linear, either.

To view it, you take the data and multiply it by the exposure level you're trying to render, then you gamma correct it. Using my e-sphere, you can set gamma in = 1.0, gamma out = 2.2, and set the luminence (exposure level) to any value you like. This way you can bring out the details in the interior, or the details in the exterior, but not both at the same time, not for direct viewing.

Reflections such as above are effectively reducing the exposure level, which means darker details become invisible and brighter details change from being impossibly white to becoming easily visible. If we were comparing exposure levels to make an HDR like the one I used above, where you have bright sun outdoors and dark indirect lighting indoors, the exposure levels to capture detail in these two areas are 1000 times apart. At the same f stop and ISO setting, I'd need to use 1/5000 of a second outdoors and 1/5 of a second indoors in order to be able to squeeze the details into the luminence data range representable by a JPEG image.

Gamma In should be 1.0 yes, but only if your HDR image is linear. Not all of them are linear. Dosch sells HDR images that are actually sRGB, which is a violation of the specification for HDR images and also a gross generalization of the sRGB color space. sRGB does not extend beyond 1. Values like 1.5, and certainly values like 150 are meaningless in sRGB.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)