Philosophy- What are numbers?

We use numbers every day, but taking a step back, what are they really? and why do they do such good job of helping us explain the universe (such as Newtonian laws)? Mathematical structures can consist of numbers, sets, groups and points- but are they real objects or do they simply describe relationships that necessarily exist in ALL structures? Plato argued that numbers were real (it doesn't matter that you can't see them) but I just thought, write down a number 6 on a piece of paper, look at it, Can you see it? well yes you can, so you can see numbers. Formalists insisted that they were merely formal systems (well-defined construction of abstract thought based on Maths). Of course most people think they know what a number is but if you try to describe one its quite hard! If anyone reading this has anything to add please comment below.