All art is abstract. It is a matter of degree, not a question of either or.
Whether one is looking at Cave Painting, Early Italian Painting, the paintings of small children, the paintings of a developed Realist or a hard edged Minimalist; abstraction is the common chord that runs through all paintings, as well as music, film, poetry, the novel, the short story and life itself.
Constructs such as realism, new realism, painterly realism, cubism, futurism, and minimalism are all attempts by humans to categorize and contain that which resists categorization and containment.
When a caveman went into the back of a cave to draw on a craggy and broken surface the image of a bison that he might meet later that day, his concern was to render that image with as much feeling and simplicity and accuracy as he could muster. Working from memory, the image that he painted on the cave wall would invariably have been a reduction, a simplification, an abstraction of the more complicated visual dynamics that are involved with replicating the image of a bison in a pre-photographic age. The caveman, like the child, is working abstractly without realizing that he is doing so.
When Vermeer painted a maiden pouring milk into a bowl amidst sunlight pouring through a dutch window, he is of necessity simplifying and abstracting forms in order to both tell his story and to reduce and simplify forms in space in a very complicated visual world. If you hold a Vermeer painting upside down and concentrate on the space between things or the space between objects — under tables, for example, and between chair legs and figures — you will see how reductive Vermeer has been. He points out to the viewer the abstract nature of space itself and the importance of the space between things. As with music — which is also fundamentally abstract — the space between things is as important as those things themselves.
Forms do not exist without space, in the same way that musical notes do not exist without the space between those notes.
Both the caveman and modern man work from an abstract construct. They may not realize that they are doing so, but that is not a concern.
If you asked 100 so-called realist painters to paint the same tree and barn and cow sitting on a green pasture from the same exact vantage point, and you had strict instructions about the overall spatial nature of the painting, you would not get 100 paintings that are exactly the same. Each artist would abstract to various degrees, simplifying this form or adjusting that space and you would have 100 different results.
Abstraction allows one to look at life in a way that is more free and clear, like space between stars on a clear dark night.