Hi M.,
Most home pregnancy tests are 99% sensitive, which means they will detect 99 pregnancies out of 100 as long as you are using the test correctly. They reach this level of accuracy about 1 week after you've missed your period, so yest -- it's a little early to see a dark line.
BTW, home tests CAN generate false-positives (pretty much every immunometric test does -- 100% specificity is rare), but it's pretty uncommon. Reasons for a false-positive result can include test background "noise" (all tests have some level of noise -- it's difference in the report between true absence and the test's limit to detect the chemical it's designed to test for), recent use of HCG-containing fertility drugs, an ectopic pregnancy, menopause, or an HCG-secreting tumor (rare!). If you're unsure about the accuracy, wait 5-6 days and take another test or get a blood test for confirmation. You can also use a different home test for confirmation as well.
HCG levels rise fairly continuously through the first 9 - 12 weeks of pregnancy and then begin to decline. The more HCG your body is producing, the darker the line will be, but only up to a point as high levels will eventually swamp out the amount of capture antibody available in the test.