Live view on my Nikon D7000 is too dark

The LCD display is not a good way to check the brightness of an image. Most camera LCDs are not perfectly calibrated, and may not have a screen brightness (NIT) that can compete with something like outdoor sunlight. For outdoors, as the sun is brighter, your pupils will close down to compensate for sunlight, but as a result, the dark objects will appear darker - that being the screen.

You can use the histogram, which is a graph of the colors in an image. Usually, an image (with varying colors) that has a histogram too far to the right is too bright. And if the graph is mostly to the left it is too dark. For an average image with varying light and dark colors, the graph should be distributed nicely through the range.

The other thing you can use is the camera's EV meter. Usually, when you press halfway down before you snap the shutter for exposure, a number may appear in the eyepiece (or it may be a line that says something like 3-2-1-0-1-2-3). This number is the camera's estimation of the exposure of an image. 0 is the middle, and numbers above, below (or on either side of the line) tell you that you may be over or underexposed by that many stops of light. Some cameras' metering settings will allow you to change the area of the frame that the camera evaluates for this exposure. But normally, this is center-averaged or mostly measures the central contents of the entire frame.

Keep in mind that these two things are simply tools. You still have to think about what you are photographing and how these tools will react. If you photographed a black sheet, the histogram may weigh everything to the left, and the EV may show up as -3. And if you shot something that was very white or extremely bright, the histogram may show up all the way to the right, and the EV may show +3.

Some people choose, based on what they are photographing, to slightly expose to the right to capture more detail in shaded or dark areas - as long as it doesn't disadvantage other areas of the image.