A second source of data for designers is web analytics. Google Analytics (GA) data is a great resource to understand the usage of your site/app as well as a bit more about who your users are. You can learn what browsers your users use, what device types (tablets, laptops, smartphones, etc.) they are accessing your content on, where in the world they are, etc. Just recently I was asked what the latest base-line monitor resolution is to design for. Armed with GA data from our own site, as well external studies, I could confidently say that the former design standard of 1024x768 has been replaced by the wider 1366x764. This wasn't a guess, or a hunch, but an informed design direction based on data.
A third source of data is the UX community. By "standing on the shoulders of giants" you can use existing research, best practices, and design standards to make smarter decisions. You can build off of the progress of others. To use another idiom, "reinventing the wheel" can be a waste of time and not all design decisions need to be groundbreaking. Sometimes the road less traveled is less traveled for a reason.
Having said all of this, it is important to remember that you shouldn't blindly follow data. I've written about data-informed decision making before. The best approach is to blend your own experience & common sense with the data to make the best decisions. User feedback, usability studies, and website analytics can all be flawed in their own ways and ultimately you need to determine the best course of action while accounting for this. Data literacy is a skill set that all designers would be better served to develop.
Design isn't as subjective as people think and with good data you can make (and defend) great design.