It is a well-known fact that in order to sustain a hypothesis in most work and knowledge fields, one needs to back it up with evidence. In science however, the aim is to disprove it. Only then could humanity move forward and develop successfully.
One of the most significant theories in biology is that cells are the smallest units of life and that all living organisms are made out of cells. However, even in cell theory there are organisms and tissues that are not made of typical cells and hence do not answer the requirements of the belief.
The first example is the Skeletal muscle. It is composed by muscle fibres and are enclosed by a membrane. However, they are much larger than the average cell and contain hundreds of nuclei.
Another example is the Giant algae (Acetabularia). Due to its big proportions, its estimated to consist of numerous small cells. In reality, its composed from one giant cell as it has only one nucleus and t=is therefore not multicellular.
The third and last example is the Aseptate Fungi. It is composed of thread-like structures called hyphae. They are not divided into sub-units, containing a single nucleus. Instead they consist of long sections, full with nuclei.
Despite these exceptions, the overall trend for living organisms being composed of cells is very strong. The cell theory has not been abandoned.