Over-extracted espresso has a dark blotchy colour with white spotting. Over-extracted espresso often flows very slowly where it will take much longer than 30 seconds to deliver 30ml of liquid. In extreme cases an over extracted expresso will often drip from the portafilter outlets.
Coffee dose is too high
Using too much coffee will result in too much resistance to the flow of water from the group, and therefore too slow a flow of water through the coffee puck.
Decrease the dose of coffee to the level that the grinder was setup for, ideally this should be near 7 grams per single shot of espresso.
Coffee grind is too fine.
Coffee that is ground too fine provides too much resistance to the flow of water from the grouphead, taking too much time to exploit the coffee.
Adjust your coffee grinder so that you achieve espresso extraction that meets the espresso extraction criteria.
Shower screen or filter is dirty/blocked
A dirty shower screen can restrict the flow of water through the grouphead causing incorrect calibration of the grind/tamp/dose.
Regular cleaning and backflushing(if possible) of your espresso machine brewing group and its components including the shower screen is vital for producing excellent espresso.
Extraction pressure is too high
Too much extraction pressure will extract undesirable compounds out of the coffee beans and into the espresso.
Too much extraction pressure is due to too much coffee being used, the grind of coffee being too fine, or too much tamping pressure used to compress the coffee. If your espresso machine has an over-pressure valve having a technician adjust this to limit the pressure to 9 bar can assist.
Water temperature is too high
Water at the point of extraction (not in the cup) that is higher than 102 degrees centigrade has a tendancy to burn the espresso resulting in over-extraction.
If your espresso machine has been sitting idle(turned on) for a long period of time, water in the grouphead can tend to get too hot. It is advisable to "flush" the brewing group by running the pump for 5 seconds with the portafilter removed to stabilise the brewing group temperature.