The concept that everything that exists is physical matter, in some or other form, is also known under the term physicalism.
It should be noted however that philosophical materialism does not claim that everything that exists is physical matter. Does a school institution exist? A state or government board? Yes, of course they exist. But, although these kind of things would not exist outside, apart and independend of physical matter, their existence form is not to be thought of as the mere physical material form in which they exist.
What kind of physical matter exist, of which a school building is built, is hardly of any relevance for the way a school institution exist. We are dealing here of course with forms of highly organized social institutions.
Also they are material entities, but don’t confuse this with physical matter.
Essential to understanding of the difference between Materialism and Idealism, is the answer to the question of what is the primary form of existence: matter or consciousness. Materialism answers to this question matter, and Idealism answers to this question consciousness.
The question now, as where to place Objectivism alongside this division between Materialism and Idealism, I think it can be clear that Objectivism can be thought of as a contemporary form of Materialism, assuming that it answers the question as to what the primary form of existence is in the same way as Materialism does.
In fact Objectivism does that, although Objectivism itself denies that, because the matter concept as Objectivism reflects to, is not the same as that of philosophical, dialectcial or historical materialism, but a very narrow matter concept. Objectivism claims that not all things that exist are material, which means the concept of matter as used within Objectivism does not cover every existing entity. Materialism claims however that everything that exist is material, i.e. they use a much broader concept of matter.
Objectivism itself claims it is it’s own category. It may say so, but could nevertheless fall into either Materialism or Idealism, dependending on how it answers the primary question in philolosophy.
It should be noted further that the term matter is in itself a confusing term, since philosophical materialism uses it in a different meaning as physics does (in physics, matter an energy are seperate things, materialism regards them both as matter, as also other existing things, like social institutions).
Instead of the concept of matter, as used in materialism (which is a broad concept, and raises above the concept of physical matter/energy alone to also include social institutions, etc), Objectism simply refers to this as ‘existence’.
Such a term can be understood as rather meaningless, or tautological, since of course, ‘existence’ exists. The materialist conception that all that exist is material, makes clear that it is an opposing viewpoint to that of Idealism.
This is however not a difference in point of view, but just a difference in terms used. The statement that ‘everything that exists is material’ in fact connects these two concepts and declares them equal.
Since I have not studied Objectivism in depth, I might be wrong on a few things, but as far as I know and understand Objectivism and Materialism, they are alltogether not much different, and much related.
Their main difference is the historical time epoch in which they have been evolved.
Philosophical and dialectical materialism has evolved some 150 years ago, although the roots of materialism must be understood to be much more ancient (Greek materialists). Objectivism is much more recent.
An important difference is their political points of view. Dialectical materialism, alongside with historical materialism, are directly linked to the class struggle, and is closely connected to the political point of view of the labour class. Objectivism on the other hand, must be considered to be the point of view of the bourgois class.
It holds to the idea that the current social system that is currently dominating the world and is the most advanced social system, is the only moral system.
That point of view is of course the point of view of the ruling classes of all previous social systems that were dominating and were the most advanced.
I.e. also the ruling class of ancient Greek slavery society, were devoted to the opinion that their society was the only moral and just social system.