Thickening of breast tissue

During a breast screening my doctor detected a "firm ill-defined non-mobile thickening" of the breast tissue in the upper outer quadrant of the right breast.  Mammograms and ultrasound were not conclusive.  Core biosies were negative.  However, my doctor was still concerned and requested an MRI which was also inconclusive but "disclosed a 9mm hypoechoic opacity at the 10 o'clock radian, 10 cm from the right nipple".  A breast surgeon examined my breasts and determined that the thickening was present on both breasts and said the thickening was "consolidation of the axillary tail of the breast".  Please describe this more fully.  What causes it and is it of any concern?

[CCF-RN,MSN-JS]

Dear NAB, The breast is tear-shaped, and there is also an area where it tapers off under the arm (axilla).  This is the area that they have described as consolidation of the axillary tail of the breast.   Consolidation is describing tissue as more solid, the concern would be whether this was cancerous

[janp645]

A related discussion, post mastectomy chest wall thickening was started.